B, the Internet.
If Caitlin told Teddy that the largest diamond was found YESTERDAY then it wouldn't be in last month's science magazine. It also wouldn't be able to be in an encyclopedia at the library because there wouldn't have been any time to write, publish, and for the library to get the encyclopedia in one day. Caitlin's parents may not know about Caitlin's claim or about the diamond in the first place. The Internet would likely have an article about the news-breaking diamond as soon as possible.