( cs ) . we provide our solutions through a software-as-a-service model , primarily with renewable annual subscriptions . these subscriptions require customers to pay a fee in order to access each of our cloud solutions . we generally invoice our customers for the entire subscription amount at the start of the subscription term , and the invoiced amounts are treated as deferred revenues and are recognized ratably over the term of each subscription . we continue to experience revenue growth from our existing customers as they renew and purchase additional subscriptions . we market and sell our solutions to enterprises , government entities and small and medium-sized businesses across a broad range of industries , including education , financial services , government , healthcare , insurance , manufacturing , media , retail , technology and utilities . in 2020 , 2019 and 2018 , approximately 63 % , 64 % and 67 % , respectively , of our revenues were derived from customers in the united states based on our customers billing address . we sell our solutions to enterprises and government entities primarily through our field sales force and to small and medium-sized businesses through our inside sales force . we generate a significant portion of sales through our channel partners , including managed service providers , value-added resellers and consulting firms in the united states and internationally . 40 impacts of covid-19 in march 2020 , the world health organization declared the outbreak of covid-19 as a pandemic . as a result of covid-19 , we have modified certain aspects of our business , including restricting employee travel , requiring employees to work from home , and canceling certain events and meetings , among other modifications . we will continue to actively monitor the situation and may take further actions that alter our business operations as may be required by federal , state or local authorities or that we determine are in the best interests of our employees , customers , partners , suppliers and stockholders . covid-19 has not had and is not expected to have a significant impact on our business in 2021. however , while we have not incurred significant disruptions from the covid-19 outbreak , we are unable to accurately predict the full impact that covid-19 will have due to numerous uncertainties , including the duration of the outbreak , actions that may be taken by governmental authorities and the impact to the business of our customers and partners . we will continue to evaluate the nature and extent of the impact to our business , financial position , results of operations and cash flows . key components of results of operations revenues we derive revenues from the sale of subscriptions to our security and compliance solutions , which are delivered on our cloud platform . subscriptions to our solutions allow customers to access our cloud-based security and compliance solutions through a unified , web-based interface . customers generally enter into one-year renewable subscriptions . the subscription fee entitles the customer to an unlimited number of scans for a specified number of devices or web applications and , if requested by a customer as part of their subscription , a specified number of physical or virtual scanner appliances . our physical and virtual scanner appliances are requested by certain customers as part of their subscriptions in order to scan it infrastructures within their firewalls and do not function without , and are not sold separately from , subscriptions for our solutions . in some limited cases , we also provide certain computer equipment used to extend our qualys cloud platform into our customers ' private cloud environment . customers are required to return physical scanner appliances and computer equipment if they do not renew their subscriptions . we typically invoice our customers for the entire subscription amount at the start of the subscription term . invoiced amounts are reflected on our consolidated balance sheets as accounts receivable or as cash when collected , and as deferred revenues until earned and recognized ratably over the subscription period . accordingly , deferred revenues represent the amount billed to customers that has not yet been earned or recognized as revenues , pursuant to subscriptions entered into in current and prior periods . cost of revenues cost of revenues consists primarily of personnel expenses , comprised of salaries , benefits , amortization of capitalized internal-use software , performance-based compensation and stock-based compensation , for employees who operate our data centers and provide support services to our customers . other expenses include depreciation of data center equipment and physical scanner appliances and computer hardware provided to certain customers as part of their subscriptions , expenses related to the use of third-party data centers , amortization of software and license fees , amortization of intangibles related to acquisitions , maintenance support , fees paid to contractors who supplement or support our operations center personnel and overhead allocations . we expect to continue to make capital investments to expand and support our data center operations , which will increase the cost of revenues in absolute dollars . operating expenses research and development research and development expenses consist primarily of personnel expenses , comprised of salaries , benefits , performance-based compensation and stock-based compensation , for our research and development teams . other expenses include third-party contractor fees , software and license fees , amortization of intangibles related to acquisitions and overhead allocations . 41 sales and marketing sales and marketing expenses consist primarily of personnel expenses , comprised of salaries , benefits , sales commissions , performance-based compensation and stock-based compensation for our worldwide sales and marketing teams . other expenses include marketing and promotional events , lead-generation marketing programs , public relations , travel , software licenses and overhead allocations . sales commissions related to new business and upsells are capitalized as an asset . story_separator_special_tag deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to tax benefit carry-forwards and to differences between the financial statement amounts of assets and liabilities and their respective tax basis . we regularly review our deferred tax assets for recoverability and establish a valuation allowance if it is more likely than not that some portion or all of the deferred tax assets will not be realized . to make this assessment , we take into account predictions of the amount and category of taxable income from available positive and negative evidence about these possible sources of taxable income . the weight given to the potential effect of negative and positive evidence is commensurate with the extent to which the strength of the evidence can be objectively verified . based on the analysis of positive and negative factors noted above , we believe it is more likely than not that our california deferred tax assets will not be realized because the income attributed to california is not expected to be sufficient to recognize these deferred tax assets . accordingly , we continue to record a valuation allowance as of december 31 , 2020 for our california deferred tax assets . if , in the future , we determine that these deferred tax assets are more likely than not to be realized , a release of all or part , of the related valuation allowance could result in an income tax benefit in the period such determination is made . w e do not have a valuation allowance against u.s. federal and certain other state deferred tax assets . we recognize an income tax expense or benefit with respect to uncertain tax positions in our financial statements that we judge is more likely than not to be sustained solely on its technical merits in a tax audit , including resolution of any related appeals or litigation processes . to make this judgment , we must interpret complex and sometimes ambiguous tax laws , regulations and administrative practices . if an income tax position meets the more likely than not recognition threshold , then we must measure the amount of the tax benefit to be recognized by determining the largest amount of tax benefit that has a greater than a 50 % likelihood of being realized upon effective settlement with a taxing authority that has full knowledge of all of the relevant facts . it is inherently difficult and subjective to estimate such amounts , as this requires us to determine the probability of various possible settlement outcomes . to determine if a tax position is effectively settled after a tax examination has been completed , we must also estimate the likelihood that another taxing authority could review the respective tax position . we must also determine when it is reasonably possible that the amount of unrecognized tax benefits will significantly increase or decrease in the 12 months after each fiscal year-end . these judgments are difficult because a taxing authority may change its behavior as a result of our disclosures in our financial statements . we must reevaluate our income tax positions on a quarterly basis to consider factors such as changes in facts or circumstances , changes in tax laws , effectively settled issues under audit , the potential for interest and penalties , and new audit activity . such a change in recognition or measurement would result in recognition of a tax benefit or an additional charge to the tax provision . 49 stock-based compensation we recognize the fair value of our employee stock options and restricted stock units over the requisite service period for those awards ultimately expected to vest . the fair value of each option is estimated on date of grant using the black-scholes-merton option pricing model and the fair value of each restricted stock unit is based on the fair value of our stock on the date of grant . forfeitures are estimated on the date of grant and revised if actual or expected forfeiture activity differs materially from original estimates . for performance-based non-qualified stock options and restricted stock units , we recognize compensation costs when it is probable that the performance conditions will be met . we assess these conditions on a quarterly basis . determining the appropriate fair value model and calculating the fair value of employee stock options requires the use of highly subjective assumptions , including the expected life of the stock option and stock price volatility . the assumptions used in calculating the fair value of employee stock options represent management 's best estimates , but the estimates involve inherent uncertainties and the application of management 's judgment . as a result , if factors change and we use different assumptions , our stock-based compensation expense could be materially different in the future . fair value measurement fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date . for certain of our financial instruments , including cash and certain cash equivalents , accounts receivable , accounts payable and accrued liabilities , the carrying amounts approximate their fair value due to the relatively short maturity of these balances . we measure and report certain cash equivalents , marketable securities and derivative foreign currency forward contracts at fair value in accordance with the provisions of the authoritative accounting guidance that addresses fair value measurements . this guidance establishes a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available . the hierarchy is broken down into three levels based on the reliability of inputs as follows : level 1 - valuations based on quoted prices in active markets for identical assets or liabilities . level 2 - valuations based on other than

liquidity and capital resources at december 31 , 2020 , our principal source of liquidity was cash , cash equivalents and marketable securities of $ 454.5 million , including $ 30.7 million of cash held outside of the united states . we do not anticipate that we will need funds generated from foreign operations to fund our domestic operations . however , if we repatriate these funds , we could be subject to foreign withholding taxes . we generated positive cash flows from operations during the years ended december 31 , 2020 and 2019. we believe our existing cash , cash equivalents , marketable securities and cash from operations will be sufficient to fund our operations for at least the next twel ve months . in 2021 , we expect capital expenditures to be in a range of $ 30.0 million to $ 35.0 million . our future capital requirements will depend on many factors , including our rate of revenue growth , the expansion of our sales and marketing activities , the timing , type and extent of our spending on research and development efforts , international expansion and investment in data centers . we may also seek to invest in or acquire complementary businesses or technologies . while the covid-19 pandemic has not had a material adverse financial impact on our operations to date , the future impact of the pandemic can not be predicted with certainty and may increase our costs of capital and otherwise adversely affect our business , result of operations , financial condition and liquidity .


 Write a concise suspicious activity report of the following text delimited by triple backquotes. Return your response as a formal report which covers the key points highlighting the location,dates,entity names and account holder information in chronological order of the text.
     ```liquidity and capital resources at december 31 , 2020 , our principal source of liquidity was cash , cash equivalents and marketable securities of $ 454.5 million , including $ 30.7 million of cash held outside of the united states . we do not anticipate that we will need funds generated from foreign operations to fund our domestic operations . however , if we repatriate these funds , we could be subject to foreign withholding taxes . we generated positive cash flows from operations during the years ended december 31 , 2020 and 2019. we believe our existing cash , cash equivalents , marketable securities and cash from operations will be sufficient to fund our operations for at least the next twel ve months . in 2021 , we expect capital expenditures to be in a range of $ 30.0 million to $ 35.0 million . our future capital requirements will depend on many factors , including our rate of revenue growth , the expansion of our sales and marketing activities , the timing , type and extent of our spending on research and development efforts , international expansion and investment in data centers . we may also seek to invest in or acquire complementary businesses or technologies . while the covid-19 pandemic has not had a material adverse financial impact on our operations to date , the future impact of the pandemic can not be predicted with certainty and may increase our costs of capital and otherwise adversely affect our business , result of operations , financial condition and liquidity .
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 Suspicious Activity Report : ( cs ) . we provide our solutions through a software-as-a-service model , primarily with renewable annual subscriptions . these subscriptions require customers to pay a fee in order to access each of our cloud solutions . we generally invoice our customers for the entire subscription amount at the start of the subscription term , and the invoiced amounts are treated as deferred revenues and are recognized ratably over the term of each subscription . we continue to experience revenue growth from our existing customers as they renew and purchase additional subscriptions . we market and sell our solutions to enterprises , government entities and small and medium-sized businesses across a broad range of industries , including education , financial services , government , healthcare , insurance , manufacturing , media , retail , technology and utilities . in 2020 , 2019 and 2018 , approximately 63 % , 64 % and 67 % , respectively , of our revenues were derived from customers in the united states based on our customers billing address . we sell our solutions to enterprises and government entities primarily through our field sales force and to small and medium-sized businesses through our inside sales force . we generate a significant portion of sales through our channel partners , including managed service providers , value-added resellers and consulting firms in the united states and internationally . 40 impacts of covid-19 in march 2020 , the world health organization declared the outbreak of covid-19 as a pandemic . as a result of covid-19 , we have modified certain aspects of our business , including restricting employee travel , requiring employees to work from home , and canceling certain events and meetings , among other modifications . we will continue to actively monitor the situation and may take further actions that alter our business operations as may be required by federal , state or local authorities or that we determine are in the best interests of our employees , customers , partners , suppliers and stockholders . covid-19 has not had and is not expected to have a significant impact on our business in 2021. however , while we have not incurred significant disruptions from the covid-19 outbreak , we are unable to accurately predict the full impact that covid-19 will have due to numerous uncertainties , including the duration of the outbreak , actions that may be taken by governmental authorities and the impact to the business of our customers and partners . we will continue to evaluate the nature and extent of the impact to our business , financial position , results of operations and cash flows . key components of results of operations revenues we derive revenues from the sale of subscriptions to our security and compliance solutions , which are delivered on our cloud platform . subscriptions to our solutions allow customers to access our cloud-based security and compliance solutions through a unified , web-based interface . customers generally enter into one-year renewable subscriptions . the subscription fee entitles the customer to an unlimited number of scans for a specified number of devices or web applications and , if requested by a customer as part of their subscription , a specified number of physical or virtual scanner appliances . our physical and virtual scanner appliances are requested by certain customers as part of their subscriptions in order to scan it infrastructures within their firewalls and do not function without , and are not sold separately from , subscriptions for our solutions . in some limited cases , we also provide certain computer equipment used to extend our qualys cloud platform into our customers ' private cloud environment . customers are required to return physical scanner appliances and computer equipment if they do not renew their subscriptions . we typically invoice our customers for the entire subscription amount at the start of the subscription term . invoiced amounts are reflected on our consolidated balance sheets as accounts receivable or as cash when collected , and as deferred revenues until earned and recognized ratably over the subscription period . accordingly , deferred revenues represent the amount billed to customers that has not yet been earned or recognized as revenues , pursuant to subscriptions entered into in current and prior periods . cost of revenues cost of revenues consists primarily of personnel expenses , comprised of salaries , benefits , amortization of capitalized internal-use software , performance-based compensation and stock-based compensation , for employees who operate our data centers and provide support services to our customers . other expenses include depreciation of data center equipment and physical scanner appliances and computer hardware provided to certain customers as part of their subscriptions , expenses related to the use of third-party data centers , amortization of software and license fees , amortization of intangibles related to acquisitions , maintenance support , fees paid to contractors who supplement or support our operations center personnel and overhead allocations . we expect to continue to make capital investments to expand and support our data center operations , which will increase the cost of revenues in absolute dollars . operating expenses research and development research and development expenses consist primarily of personnel expenses , comprised of salaries , benefits , performance-based compensation and stock-based compensation , for our research and development teams . other expenses include third-party contractor fees , software and license fees , amortization of intangibles related to acquisitions and overhead allocations . 41 sales and marketing sales and marketing expenses consist primarily of personnel expenses , comprised of salaries , benefits , sales commissions , performance-based compensation and stock-based compensation for our worldwide sales and marketing teams . other expenses include marketing and promotional events , lead-generation marketing programs , public relations , travel , software licenses and overhead allocations . sales commissions related to new business and upsells are capitalized as an asset . story_separator_special_tag deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to tax benefit carry-forwards and to differences between the financial statement amounts of assets and liabilities and their respective tax basis . we regularly review our deferred tax assets for recoverability and establish a valuation allowance if it is more likely than not that some portion or all of the deferred tax assets will not be realized . to make this assessment , we take into account predictions of the amount and category of taxable income from available positive and negative evidence about these possible sources of taxable income . the weight given to the potential effect of negative and positive evidence is commensurate with the extent to which the strength of the evidence can be objectively verified . based on the analysis of positive and negative factors noted above , we believe it is more likely than not that our california deferred tax assets will not be realized because the income attributed to california is not expected to be sufficient to recognize these deferred tax assets . accordingly , we continue to record a valuation allowance as of december 31 , 2020 for our california deferred tax assets . if , in the future , we determine that these deferred tax assets are more likely than not to be realized , a release of all or part , of the related valuation allowance could result in an income tax benefit in the period such determination is made . w e do not have a valuation allowance against u.s. federal and certain other state deferred tax assets . we recognize an income tax expense or benefit with respect to uncertain tax positions in our financial statements that we judge is more likely than not to be sustained solely on its technical merits in a tax audit , including resolution of any related appeals or litigation processes . to make this judgment , we must interpret complex and sometimes ambiguous tax laws , regulations and administrative practices . if an income tax position meets the more likely than not recognition threshold , then we must measure the amount of the tax benefit to be recognized by determining the largest amount of tax benefit that has a greater than a 50 % likelihood of being realized upon effective settlement with a taxing authority that has full knowledge of all of the relevant facts . it is inherently difficult and subjective to estimate such amounts , as this requires us to determine the probability of various possible settlement outcomes . to determine if a tax position is effectively settled after a tax examination has been completed , we must also estimate the likelihood that another taxing authority could review the respective tax position . we must also determine when it is reasonably possible that the amount of unrecognized tax benefits will significantly increase or decrease in the 12 months after each fiscal year-end . these judgments are difficult because a taxing authority may change its behavior as a result of our disclosures in our financial statements . we must reevaluate our income tax positions on a quarterly basis to consider factors such as changes in facts or circumstances , changes in tax laws , effectively settled issues under audit , the potential for interest and penalties , and new audit activity . such a change in recognition or measurement would result in recognition of a tax benefit or an additional charge to the tax provision . 49 stock-based compensation we recognize the fair value of our employee stock options and restricted stock units over the requisite service period for those awards ultimately expected to vest . the fair value of each option is estimated on date of grant using the black-scholes-merton option pricing model and the fair value of each restricted stock unit is based on the fair value of our stock on the date of grant . forfeitures are estimated on the date of grant and revised if actual or expected forfeiture activity differs materially from original estimates . for performance-based non-qualified stock options and restricted stock units , we recognize compensation costs when it is probable that the performance conditions will be met . we assess these conditions on a quarterly basis . determining the appropriate fair value model and calculating the fair value of employee stock options requires the use of highly subjective assumptions , including the expected life of the stock option and stock price volatility . the assumptions used in calculating the fair value of employee stock options represent management 's best estimates , but the estimates involve inherent uncertainties and the application of management 's judgment . as a result , if factors change and we use different assumptions , our stock-based compensation expense could be materially different in the future . fair value measurement fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date . for certain of our financial instruments , including cash and certain cash equivalents , accounts receivable , accounts payable and accrued liabilities , the carrying amounts approximate their fair value due to the relatively short maturity of these balances . we measure and report certain cash equivalents , marketable securities and derivative foreign currency forward contracts at fair value in accordance with the provisions of the authoritative accounting guidance that addresses fair value measurements . this guidance establishes a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available . the hierarchy is broken down into three levels based on the reliability of inputs as follows : level 1 - valuations based on quoted prices in active markets for identical assets or liabilities . level 2 - valuations based on other than

