So (D) works because it investigates whether there is some (non-mosquito) reason that the monkeys do this behavior in the rainy season.
(E) doesn’t work because of one thing: “other insects” are irrelevant to the question of why the monkeys use the millipedes. You made some assumptions when arguing that if other insects are available, the continued use of millipedes indicates the monkey’s preference for millipedes. As you've seen before, it's not necessarily that your assumptions were "wrong," it's that making assumptions is just not allowed, as you wouldn't even be able to verify "right" assumptions. Even if it were true, that “preference” for millipedes doesn’t indicate that repelling mosquitos is the only goal of the bug-rubbing behavior.
TL;DR: Don’t make assumptions about what Yes/No responses indicate. The right answer needs to be directly related to the conclusion in a more obvious way, and should not require so much thinking.
Also, consider writing down the Conclusion/Question paraphrase first, as I did above:
I'm not sure, and apologies if I'm misinterpreting, but I think your focus may have drifted to “What are ways the monkeys can repel mosquitos? Are millipedes best?” instead of "Is repelling mosquitos the only goal of the behavior?"JbhB682 Wrote:I thought this hypothesis was strengthened by eliminating other ways monkey can repeal the mosquito.