If you choose D, the first verb is an action verb. This adverb seriously modifies the smell of verbs. As an adverbial, smooth bad has a poor sense of smell.

The second smeets is a copulative verb, followed by an adjective as a predicative, smells bad smells bad.

It doesn't matter if the hound's sense of smell is not sensitive. What I care about is whether it gives off an unpleasant smell.

If you choose C, the first "smeets" is a verb, followed by an adjective as a predication, and "smeets bad" smells bad.

The second smell is a behavioral verb; This adverb seriously modifies the smell of verbs. As an adverbial, smooth bad has a poor sense of smell.

It doesn't matter if the hound smells bad, what I care about is whether its sense of smell is sensitive.