This would not strengthen the authors’ argument. Remember that the authors argue that earlier migrants paved the way for later migrants, partially by providing information about things like labor markets. If it took longer for later migrants to get jobs, then the information in (A) shows one way that earlier migrants actually weren’t able to help later migrants establish themselves in the North.

(B) supports the authors’ hypothesis about the momentum of migration. The authors argue that earlier migrants paved the way for later migrants by sending information back to their home communities in the South. As a result, when people from those communities decided to migrate north, they were more likely to follow in the footsteps of the earlier migrants. Doing so allowed them to benefit from the support and familiarity offered by individuals from their own community who had already settled in the North. Thus, if the authors’ theory is correct, we would expect for communities of African Americans in the North to consist largely of individuals who came from the same place in the South, so (B) supports this argument.

This doesn’t impact the authors’ argument. The authors’ argument is that the Great Migration continued for decades because, overall, connections with previous migrants reduced the difficulty of migrating for those who came later; comparing the stability of housing prices in the North and the South doesn’t impact this argument.

This suggests an alternative hypothesis to explain why the Great Migration continued––according to (D), the continuation was not necessarily because of the momentum of migration (which is the authors’ argument), but instead could be because of increased efforts from Northern employers to recruit Southern labor. Because this suggests an alternative hypothesis, this actually weakens the author’s argument.

The authors’ argument explains why the Great Migration continued until 1960, so a large-scale reverse migration that occurred later in the 20th century doesn’t impact the authors’ argument.