It only discusses one possible alternative in the last passage, but the primary concern of the passage is "Assessing possible alternative explanations" as in plural. If it was only assessing one possible explanation then i could see it. However, I do not see how you would get that answer. If someone could elaborate, It'd be appreciated
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Hi @kyland, sometime with main point or primary purpose questions such as this, the correct answer choice will feel somewhat unsatisfying. I agree that the language of "alternative explanations" is a bit puzzling at first. I think in those case it means that the author is assessing two (or more) possible explanations for the die off. This actually does match what we saw in the passage. The author begins by discussing a generally accepted hypothesis for why the die off happens, only to reject that hypothesis in favor of their own explanation regarding pollution.
That said, let's take another look at why D is right, or, why the other 4 are not right.
The passage is structured with the author introducing a point of view and its supports. That is, the PCB hypothesis. In the final paragraph, the author rejects this view, explains why they reject it, and then proposes a view of their own.
A is a much too narrow scope. The idea of bacterial infections is only one relatively minor part of the passage.
B is factually incorrect. The passage does not actually discuss the process by which the correct diagnosis was reached.
C looks tempting, but misrepresents the passage. The author does not actually attack the research that went into the PCB hypothesis, just the hypothesis itself.
E also misrepresents the passage. It does not compare the impact of various toxins.
Therefore, while D is fairly bland, the passage does evaluate two alternative explanations.
