In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: BACKGROUND : Osteoporosis is caused by bone resorption in excess of bone formation , and bisphosphonates , are used to inhibit bone resorption .
### Output Text:
BACKGROUND : Osteoporosis is caused by bone resorption in excess of bone formation , and <span class="chemical">bisphosphonates</span> , are used to inhibit bone resorption . 