Not quite getting this question

Hello, can you please explain how D was the answer? Thank you in advance.

@rmkrutz@crimson.ua.edu @Ro13 The "two streams" are mentioned in relation to the changing demand side, as noted at the top of the paragraph. The paragraph goes into the two options created in the current market for immigrants: so-called skilled versus unskilled labor, creating two "streams." This is what D tells us, which is why it is correct.