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题库 / Prep2008E2-CR-92
Kate: The recent decline in numbers of the Tennessee warbler, a North American songbird that migrates each fall to coffee plantations in South America, is due to the elimination of the dense tree cover that formerly was a feature of most South American coffee plantations.
Scott: The population of the spruce budworm, the warbler's favorite prey in North America, has been dropping. This is a more likely explanation of the warbler's decline.
Which of the following, if true, most seriously calls Scott's hypothesis into question?
The numbers of the Baltimore oriole, a songbird that does not eat budworms but is as dependent on South American coffee plantations as is the Tennessee warbler, are declining.
The spruce budworm population has dropped because of a disease that can infect budworms but not Tennessee warblers.
The drop in the population of the spruce budworm is expected to be only temporary.
Many Tennessee warblers have begun migrating in the fall to places other than traditional coffee plantations.
Although many North American songbirds have declined in numbers, no other species has experienced as great a decline as has the Tennessee warbler.
A减少了，B也减少了，最终C减少了，到底是A引起的C减少，还是B引起的C减少呢？
有一个D，D和A不相关，D和B相关，现在D也减少了。
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