You can see (A) through either lens – it bolsters the link between the poison fungus and moth deaths by showing that when you take the poison factor away (caterpillars are unaffected by the fungus), a similarly situated moth strain still thrives.

It also eliminates alternative explanations that involve all moths dying for some other reason (like the rise of a moth-chomping bat or something).

What happened with those other species, though? To strengthen our argument, (B) would need to add something like “those few insect species also declined, while all the other species rose.” As it stands, the mere existence of other poisoned/unpoisoned species doesn’t tell us anything.

But good job, entomologists – you made the noble decision not to carpet-bomb the whole forest. Here’s a cookie.

This one’s super wack. An increase in moths? Are we talking about the original cause of the moth problem back in the day? If so, that’s wildly irrelevant to our current argument about what caused the recent decrease in moths.

Even squinting at (C), though, all it does is suggest an alternate cause (predators) for changes in moth populations. This is a strengthen question – we’re looking to eliminate alternate causes.

This weakens the argument by introducing an alternate cause – maybe the moths are dying because the oak trees are dying and the moths depend on oak leaves for food!

This is a strengthen question.

This weakens the argument by introducing evidence of a similar phenomenon occurring (moth populations dropping) in an environment where the proposed cause isn’t present (no fungus in the other forests).

This is a strengthen question.