Humans become part of the carbon cycle when they A. breathe in carbon dioxide,
B. release carbon as a waste product,
C. produce new carbon compounds in their cells,

Human activities have a tremendous impact on the carbon cycle

Explanation:

Burning fossil fuels, changing land use, and using limestone to make concrete all transfer significant quantities of carbon into the atmosphere. ... The ocean absorbs much of the carbon dioxide that is released from burning fossil fuels.

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.