When the liquid is poured through a fliter,

These impurities remain intact to the filter and they get separated out from the liquid. Hence, when we pour a liquid through a filter then only the liquid passes through the filter and rest of the impurities if present in the liquid remain in the filter and doesn't passes through it

Explanation:

the answer is b because, investigating whether another species of moth also drinks tears might be interesting, but it would not directly explain why this species of moth does.

to explain why this species of moth drinks tears, the scientist could ask: "what nutrients that moths need are in elephant tears? "

the scientist could experiment with moth repellent or oter ways to keep the moths away, but that would not answer his original question.