Quindlen links the conclusion to the introduction of her essay with the words "like many improbable ideas, when it works, it's a wonder", which is very close to her claim in the overall text, "America is an Improbable idea", it isn't supposed to work but it does despite all of the differences, specifically racial or ethnic, within it.
It is D, because if you saw a simple sentence that said, "They rule the earthly world of men together," you wouldn't know who "They" are. That is why it is dependent, because it is depending on the rest of the paragraph to do its part first. :D
