A、The number of visits they receive. B、The way they store data. C、The files they have collected. D、The means they use to get inf

It’s always fun to write about research that you can actually try out for yourself. Try this: Take a photo and upload it to Facebook, then after a day or so, know what the URL link to the picture is and then delete it. Come back a month later and see if the link works. Chances are, it will. Facebook isn’t alone here. Researchers at Cambridge University have found that nearly half of the social networking sites don’t immediately delete pictures when a user request they be removed. In general, photo-centric websites like Phlica were found to be better at quickly removing deleted photos upon request. Why do deleted photos stick around so long? The problem relates to the way data is stored on large websites. While your personal computer only keeps one copy of a file, large-scale services, like Facebook, rely on what are called Content Delivery Networks to manage data and distribution. It’s a complex system wherein data is copied to multiple intermediate devices, usually to speed up access to files when millions of people are trying to access the service at the same time. But because changes aren’t reflected across the Content Delivery Networks immediately, ghost copies of files tend to linger for days or weeks. In the case of Facebook, the company says data may hang around until the URL link in question is reused, which is usually after a short period of time, though obviously that time can vary considerably.
Questions 13 to 15 are based on the passage you have just heard.
13. What does the speaker ask us to try out?
14. What accounts for the failure of some websites to remove photos immediately?
15. When will the unwanted data eventually disappear from Facebook according to the company?