Because it is a book with answers to many other questions like the fastest bird in Europe。
句意:为什么比弗爵士认为吉尼斯世界纪录是个好主意?
根据文章第二段Sir Beaver thought that there must be many other questions like this, but there was no way to find the answer. He realized that a book with answers like this may be a good idea. He was right!(爵士想到一定还有很多这样的问题,但是没有办法找到答案。他意识到一本有这样答案的书可能是个好主意。他是对的!)可知,原因是这本书有很多答案。故填
Because it is a book with answers to many other questions like the fastest bird in Europe.