B is the best answer. To answer this question, you must first identify the author’s argument. The author argues that it is possible that Black migrants to the North were living and working in urban areas of the South rather in rural areas, as researchers had previously assumed. In lines 44-48, the author states that it may be “surprising” that an employed population would relocate. Thus, the author anticipates an objection to her argument on the grounds that Black urban workers in the South would have been unlikely to leave an economically secure existence. She meets that objection by stating that “an explanation lies in the labor conditions then prevalent in the South” (lines 46-48), and discusses the low wages that may have motivated Black workers to migrate north for higher pay.