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题库 / Prep2012-SC-64
Although beavers can still be found throughout the western United States, their number has declined compared with the eighteenth century, when the abundance of beavers lured armies of European trappers into the Rocky Mountain region to slaughter them for their pelts.
their number has declined compared with
their numbers have declined compared with
their numbers have declined since
their numbers have in comparison to those of
the number of them declined since
题目分析：
在(A)(B)(D)中，their number has declined和the eighteenth century之间是对比关系（compared with）；在(C)(E)中，their number has declined和the eighteenth century之间是自从关系（since）。在逻辑上，与划线部分形成转折关系（although）的内容是：“在美国的西部海狸依然能被发现（beavers can still be found throughout the western United States）”，如果想和前文形成转折关系，那么划线部分的内容应该表述为“海狸的数量自从18世纪就开始下降了”而不是“相比于18世纪海狸的数量，现在海狸的数量下降了”（如果仅仅是数量下降话，这反而证明海狸应该能被发现（只是下降，没有消失），两个小句不会存在转折关系），可以排除(A)(B)(D)。第二，如果时间状语是由since引导，那么谓语动词必须以完成时的形式出现，可以排除(E)，答案为(C)。
选项分析：
略
某物的数量到底是多还是少并不清楚时，倾向于用numbers，如：
In Scotland, the wild salmon’s numbers have been reduced by uncontrolled deep-sea and coastal netting, by pollution, and by various other threats to the fish’s habitat.三文鱼的数量虽然是一个数字，也用了numbers。
The gyrfalcon, an Arctic bird of prey, has survived a close brush with (A) extinction; its numbers are now five-times greater than when the use of DDT was sharply restricted in the early 1970’s.北极鸟的数量用了numbers
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