The answers are C and D Good is not a noun, and thus it cannot be a subject, because it is an adjective. B - gardeners, is a noun, but not a subject, it is the object of this sentence. C is the correct answer, it is a pronoun, which substitutes a noun, so they basically mean the same. And D is also correct, because it is the "doer of the action", it is falling. You can easily find the subject by asking the question - Who is doing it?