2009托福官方指南阅读6篇.doc

Reading Practice Sets PRACTICE SET 1 THE ORIGINS OF CETACEANS 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 fluke1 and blowhole2 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? It clearly indicates that cetaceans are mammals. It cannot conceal the fact that cetaceans are mammals. It is the main difference between cetaceans and land-dwelling mammals. It cannot yield clues about the origins of cetaceans. 2. Which of the following can be inferred from paragraph 1 about early sea otters? It is not difficult to imagine what they looked like. There were great numbers of them. They lived in the sea only. They did not leave many fossil remains. The fossil consists of a complete skull of an archaeocyte, an extinct group of ancestors of modern cetaceans. Although limited to a skull, the Pakicetus fossil provides precious details on the origins of cetaceans. The skull is cetacean-like but its jawbones lack the enlarged space that is filled with fat or oil and used for receiving underwater sound in modern whales. Pakicetusprobably detected sound through the ear opening as in land mammals. The skull also lacks a blowhole, another cetacean adaptation for diving. Other features, however, show experts that Pakicetusis a transitional fo