A compound contained 1000g of element X when it was buried. Element X is radioactive(half-life=200 years) If this compound is dug up in 1400 years, how many grams of Element X would you expect to find?

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.

the answer your looking for is c).