A30-32题-英语听力

[00:00.97]Passage two [00:03.24]It's always fun to write about research that you can actually try out for yourself. [00:09.23]Try this: Take a photo and upload it to Facebook, then after a day or so, [00:16.88]note what the URL link to the picture is and then delete it. [00:21.67]Come back a month later and see if the link works. [00:26.04]Chances are: It will. Facebook isn't alone here. [00:32.25]Researchers at Cambridge University have found that nearly half of the social networking sites don't immediately delete pictures [00:40.35]when a user requests they be removed. [00:43.45]In general, photo-centric websites like Flickr were found to be better at quickly removing deleted photos upon request. [00:53.68]Why do "deleted" photos stick around so long? [00:57.46]The problem relates to the way data is stored on large websites: While your personal computer only keeps one copy of a file, [01:07.87]large-scale services like Facebook rely on what are called content delivery networks to manage data and distribution. [01:17.43]It's a complex system wherein data is copied to multiple intermediate devices, [01:23.40]usually to speed up access to files when millions of people are trying to access the service at the same time. [01:31.59]But because changes aren't reflected across the content delivery networks immediately, [01:37.15]ghost copies of files tend to linger for days or weeks. [01:41.60]In the case of Facebook, the company says data may hang around until the URL in question is reused, [01:49.99]which is usually "after a short period of time",though obviously that time can vary considerably.