[求助]GWD-8-3

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 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.

请教A是怎么weaken的？别的选项又不象。想不明白

      the passage tells us that two factors can lead to the decrease of  the population of sea otters. furthermore, the passage precludes the effect of one factors (killer whales) by presenting a fact that the population of killer whales is unchanged.

       if the population of killer whales is unchanged and if their primary foods such as seals and sea lions largely decreased, as mentioned in the passage, killer whales will certainly eat sea otters to sustain their lives. thus choice A provides another reason to explain the declining of the number of sea otters.