While this is stated in the passage, this isn’t the main point. The main point of the passage is to explore why the Great Migration continued, not just to define the Great Migration. This is just the phenomenon; the main point will include the phenomenon and the hypothesis that explains it.

While this is stated in the passage, this is not the main point. (B) just explains why the Great Migration began. Instead, the main point is to explore why the Great Migration continued for decades, even as North-South income differences narrowed.

(C) does capture the phenomenon (the continuation of the great migration), but (C) gives an incomplete explanation of the authors’ hypothesis. The fact that earlier migrants helped reduce the financial costs of migration for later migrants is part of the hypothesis, but not the whole thing. The authors’ hypothesis is also that previous migrants helped ease the transition for later migrants by providing information, temporary housing, food, credit, and a cultural cushion. Because (C) only mentions the financial aspect, it doesn’t fully capture the authors’ hypothesis and it isn’t the main point of the passage.

While this is stated in the passage, this is not the main point. The main point is to explore Great Migration specifically and to hypothesize why it continued for decades; (D) doesn’t even mention the Great Migration and instead makes a broad claim about migration movements in general.