It should be obvious that cetaceans-whales, porpoises, and dolphins-are mammals. They breathe through lungs, not through gills, and give birth to live young. Their streamlined bodies, the absence of hind legs, and the presence of a fluke3 and blowhole4 cannot disguise their affinities with land-dwelling mammals. However, unlike the cases of sea otters and pinnipeds (seals, sea lions, and walruses, whose limbs are functional both on land and at sea), it is not easy to envision what the first whales looked like. Extinct but already fully marine cetaceans are known from the fossil record. How was the gap between a walking mammal and a swimming whale bridged? Missing until recently were fossils clearly intermediate, or transitional, between land mammals and cetaceans.
1. In paragraph 1, what does the author say about the presence of a blowhole in cetaceans?
o It clearly indicates that cetaceans are mammals.
o It cannot conceal the fact that cetaceans are mammals.
o It is the main difference between cetaceans and land-dwelling mammals.
o It cannot yield clues about the origins of cetaceans.
事实推断题，先找到原句，然后找最贴近原文的叙述。我们可以看到can't disguise their affinites with land-dwelling mammals和第二个选项canceale the fact that...最为贴近。