Television is a relatively stable advertising medium. In many ways, the television ads today are almost the same to those two decades ago. Most television ads still feature actors, still run 30 or 60 seconds, and still show a product. However, the different medium of the Internet causes unique challenges to advertisers, forcing them to adapt their practices and techniques. In the early days of Internet marketing, online advertisers used banner   and pop-up ads   to attract customers. These techniques reached large audiences, led to many sales leads, and came at a low cost. However, a small number of Internet users began to consider these advertising techniques annoying. Yet because marketing strategies relying heavily on banners and pop-ups produced results, companies invested growing amounts of money into purchasing these ad types. As consumers became more complicated, frustration with these online advertising techniques grew. Independent programmers began to develop tools that blocked banner and pop-up ads. A major development in online marketing came with the introduction of pay-per-click ads. Unlike banner or pop-up ads, which originally required companies to pay every time a website visitor saw an ad, pay-per-click ads allowed companies to pay only when an interested potential customer clicked on an ad. More importantly, however, these ads are not affected by the pop-up and banner blockers. As a result of these advantages and the incredible growth in the use of search engines, which provide excellent places for pay-per-click advertising, a great number of companies began turning to pay-per-click marketing. However, as with the banner and pop-up ads, pay-per-click ads came with their shortcomings. When companies began pouring billions of dollars into this emerging medium, online advertising specialists started to notice the presence of what would later be called click fraud  : representatives of a company with no interest in the product advertised by a competitor click on the competitor's ads simply to increase the marketing cost of the competitor. Click fraud grew so rapidly that marketers sought to diversify   their online positions away from pay-per-click marketing through new mediums. Although pay-per-click advertising remains a common and effective advertising tool, marketers adapted yet again to the changing elements of the Internet by adopting new techniques such as pay-per-performance advertising. As the pace of the Internet's evolution increases, it seems all the more likely that advertising successfully on the Internet will require a strategy that avoids constancy   and welcomes change. As an advertising medium, the television and the Internet mainly differ in   _  .
the type of individual each medium reaches whether the medium is interactive the pace at which the medium develops the cost of advertising with each medium