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?
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AKiller whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.
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BThere is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980's.
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CAlong 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.
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DFollowing 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.
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EThe North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.
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正确答案: A