The course of a chemical reaction can be traced by using a a) polar molecule b) diatomic molecule c) stable isotope d) radioisotope

D radioisotopes

Explanation: radioactive isotopes are used because they emits radiation. With these radiations, scientists can trace the course of a chemical reaction

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.