The mass of an atom is found by adding its protons and (space) the electric charge of an atom is found by adding the charges carried by its protons and (space)

(first space)
1.electrons
2.neutrons
3.nucleus
(second space)
1.electrons
2.neutrons
3.nucleus

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.