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?

1. 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.

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

3. 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.

4. 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.

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

为什么选A呢？实在是想不明白。

作者说两个原因导致数量下降

1鲨鱼

2病

又说应该是病占主要原因

因为After all, a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution-related disease

但是鲨鱼数量没变

题目weakens the reasoning

A说是鲨鱼是主要原因

sea otters 减少有两个原因：1.killer whales 多了。2.疾病

推理：因为一种污染造成的疾病使得两个和（sea otter无关的东西）海狮和海豹减

少了，这样的疾病可能会影响sea otters。所以是疾病影响了数量

A说的是killer whales当食物来源减少了（海狮/海象）就会去吃其它的东西。

就是说，killer whales由于海狮/海象烧了，会去吃sea otter。使得其数量减少