实在是头大了。我选了，Ｅ

答案是A。。。请NN们帮我解释一下把、

建议楼主先把题目给大家贴一下，这样方便大家给你回贴，同时回贴速度也会加快的！

GWD-8-Q3:

In the late 1980’s, the population of sea otters in the North Pacific Ocean began to decline. Of the two plausible explanations for the decline—increased predation by killer whales or disease—disease is the more likely. After all, a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution-related disease, which could have spread to sea otters, whereas the population of killer whales did not change noticeably.

Which of the following, if true, most seriously weakens the reasoning?

A. Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.

B. There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980’s.

C. Along the Pacific coast of North America in the 1980’s, sea otters were absent from many locations where they had been relatively common in former times.

D. Following the decline in the population of the sea otters, there was an increase in the population of sea urchins, which are sea otters’ main food source.

E. The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.[A]

另外建议楼主在发新贴前先搜索一下，好东东多多噢！

8-3             http://forum.chasedream.com/dispbbs.asp?BoardID=24&ID=76580

8-3             http://forum.chasedream.com/dispbbs.asp?BoardID=24&ID=70019

这题的必要条件是After all, a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution-related disease，就是说疾病导致了那个海象和海豹的减少。并导致了 sea otters 水濑 的减少，这个水濑吃海象和海豹，由于那两个冬冬少了，所以导致一个逻辑链，疾病-->食物减少--〉水濑少了，削弱的话就是，水濑当然还是可以用其他食物的咯。A给了一个反例，

不知道你懂了吗