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?
The answer is the midpoint because halfway through our stories, something marvelous happens. you have to keep on going to get to the point where the author wants you to get to.
