D is the best answer. The first paragraph describes a common assumption about the Great Migration, that the majority of migrants came from rural areas. It also restates the conclusion that is based on this assumption, that the subsequent economic difficulties of Black migrants in the North were a result of their unfamiliarity with urban life. In the second paragraph, the author states that the “question of who actually left the South” (lines 27-28) has never been adequately researched. She goes on to argue that Black migrants may actually have been from urban areas rather than rural areas, and thus that their subsequent economic problems in northern cities were not caused by their rural background. In making this argument, the author is challenging the “widely accepted explanation” presented in the first paragraph.