The correct answer is definitely choice A
The argument is:
Disease, rather than predation, was more likely the cause of the decline in sea otters. (In abstract: X rather than Y is more likely the cause of Z).
To weaken the argument, we need to find a choice that says that predation is the more likely cause or at least a lot more likely than the author thought. (In abstract: a choice that says Y is a very likely cause of Z).
Choice A tells us that killer whales (a predator) are likely to eat sea otters when there aren't seals or sea lions around. So, when the seal and sea lions' numbers were shrunk by disease, it is very plausible that the killer whales started eating the sea otters. Thus, the sea otters' population decline may indeed be attributable to predation (rather than disease).
Choice B rules out a third explanation--migration. If there are 3 competing explanations for a phenomenon, and 1 gets ruled out, then of course the other 2 explanations are collectively more likely. However, neither of the two remaining explanations relative to one another is more likely. And since the argument is that one explanation (disease) is more likely than the other (predation), this choice is neutral, and fails to weaken.
Choice C is irrelevant; if anything it is a natural consequence of the decline in their population. However, affirming that there was a decline doesn't make it any more or less likely that disease (rather than predation) was the cause.
Choice D is irrelevant, and like choice C, it is a natural consequence or implication of the sea otters' population decline.
Choice E tends to break the connection between {sea lions + seals} and sea otters. Thus, this answer choice invites the test-taker to devalue the stated evidence of the argument, to wit, that diseases that affect {sea lions + seals} are also likely to affect sea otters. However, we must always accept stated evidence as true. Thus, this answer choice is irrelevant.
Kaplan Teacher in Toronto
