In the late 1980s, the population of sea otters in theNorth Pacific began to decline. Thereare two plausible explanations for the decline: predation, possibly by killer whales, or disease. Of these two, disease is the more likely,since a concurrent sharp decline in populations of seals and sea lions isbelieved to have been caused by disease, and diseases that infect thesecreatures are likely to be able to infect sea otters also.
Which of the following, if true, most seriously weakensthe reasoning?
(A)Killer whales in the North Pacific usually prey on seals and sea lions butwill, when this food source is scarce, seek out other prey.
(B) There is no indication that the sea otter populationat any North Pacific location declined in the 1980s because of substantialnumbers of sea otters migrating to other locations.
(C) Along the Pacific coast of North America in the1980s, sea otters were absent from many locations where they had beenrelatively common in former times.
(D) Following the decline in the population of the seaotters, there was an increase in the population of sea urchins, which are seaotters' main food source.
(E) The North Pacific populations of seals and sea lionscover a wider geographic area than does the population of sea otters.
参考答案: A
思路：现象：In the late 1980s, thepopulation of sea otters in the North Pacific began to decline.
原因：disease is the morelikely.
正确答案作用：不是这个原因，应该是另外的原因。
思考：文中给出了两个可能的原因，如果不是disease这个原因，那就应该是predation by whales这个原因。所以答案应该就是在表明是predation by whales这个原因。
选项分析：B：没有证据说明“sea otter的下降是由于migrating to otherlocations”。这个选项是否定掉了migrating这个原因。我们需要的答案是表明是predation by whales这个原因。并且这是否定句;
C：这个选项加强了“现象”;
D：在seaotter数量下降之后, sea urchins的数量上升. 这个选项和我们需要的”sea otter数量下降是由于predation by whales”不相符合;
E：Seals和sea lion比起sea otter覆盖了更多的地方, 这个与sea otter下降没有关系;
A: 此选项与whales相关,需要重点关注. whales在sealsand sea lions不够的时候,会捕食别的东西. 根据文中叙述, seals and sea lions由于疾病影响下确实在下降, 所以whales会去捕食sea otter. 正好说明sea otter的下降是由于whales, 而不是disease. 削弱了现象的解释, 正确答案.
