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