Questions are based on the recording you have just heard.
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[16] We know whales are mammals and that they evolved from land creatures. So the mystery_ is figuring out how they became ocean dwellers. Because until recently there was no fossil record of what we call "the missing link" -- that is evidence of species that show the transition between land-dwelling mammals and today's whales. Fortunately, some recent fossil discoveries have made the picture a little bit clearer. For example, a few years back in Pakistan, they found a skull of a wolf-like creature. The ear area of the skull had characteristics seen like, specifically the Whales'. Err, well, then also in Pakistan, they found fossils of other creatures, like the Ambulocetus natans which means "walking whale that swims". And very recently in Egypt, they found a skeleton of Basilosaurus. It has been linked to modem whales because of its long whale-like body. But they are still very different from the whales who know today. So really we are working in just a few pieces of a big puzzle.
Emm... another related debate involves some recent DNA studies. Remember, [17] DNA is the genetic code for any organism. And when the DNA from two different species is similar, it suggests that those two species are related. And when we compared some whale DNA with DNA from some other species, we got quite a surprise.[17] The DNA suggests the whales are descendants of the hippopotamus. Yes, the hippopotamus! Well, it came as a bit of a shock. I mean, that a four-legged land and fiver dweller could be the evolutionary source of a completely creature living in water and up to 25 times its size? Unfortunately this revelation about the hippopotamus apparently contradicts the fossil record, which suggests that the hippopotamus is only a very distant relative of the whale, not an ancestor. And of course as I mentioned, that whales are descendant not from hippos but from that distant wolf-like creature. [18] So we have contradictory evidences. And more research might just raise more questions and create more controversies. At any rate, we have a choice. We can believe the data -- the DNA, or we can believe the skeleton trail, but unfortunately, probably not both.
Err... and there have been some other interesting findings from DNA research. For a long time, we assumed that all whales that had teeth including sperm whales and killer whales were closely related to one another. And the same for the toothless whales, like the blue whale and other baleen whales, we assumed that they'd be closely related. [19] But recent DNA studies suggest that that's not the case at all. The sperm whale is actually closely related to the baleen whale, and it's only distantly related to the toothed whales. So that was a real surprise to all of us.
【预测】选项均为概况性较强的名词短语表明，本题可能考查录音的主旨。
【解析】B。录音中教授提到，我们知道鲸鱼是从陆地生物(land creatures)进化而来的哺乳动物，那谜团就是它们是如何变成海洋生物的(ocean dwellers)，直到最近我们都没有找到任何被称为"丢失的一环"的化石记录。接着介绍了针对这个谜团的一些化石发现和DNA研究，但并没有得出一致的结论。由此可知，本段录音意在表明目前还很难确定鲸鱼的进化史，故答案为B。