D
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?
A: There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980's.
B: 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.
C: The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.
D: 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.