Which compound contains nitrogen with an oxidation number of +3?

Nitrous Acid

Nitrogen is in the +3 oxidation state in N2O3. Nitrous acid is a weak acid.

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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.