Roughly 40 percent of the African American population of the Southern United States left the South between 1915 and 1960, primarily for the industrial cities of the North. While there was some African American migration to the North during the nineteenth century, most accounts point to 1915 as the start of what historians call the Great Migration. There were at least three catalysts of the Great Migration. First, World War I increased labor demand in the industrial North. Second, the war in Europe cut off immigration, which led many Northern employers to send labor agents to recruit African American labor in the South. Finally,a boll weevil infestation mined cotton crops and reduced labor demand in much of the South in the 1910s and 1920s. In short, the Great Migration began in 1915 and not earlier, because it was only then that the North-South income gap became large enough to start such a large-scale migration. Less dear, however, is why migration continued, and even accelerated, in subsequent decades, at the same time that North-South income differences were narrowing. We propose that once started, migration develops momentum over time as current migration reduces the difficulty and cost of future migration. Economists have typically assumed that people migrate if then- expected earnings in the destination exceed those of the origin enough to outweigh the difficulties and one-time costs of migration. Previous research suggests that the difficulties and costs arise from several sources. First, the uncertainty that potential migrants face concerning housing and labor-market conditions in the destination presents a significant hindrance. Second, there is the simple cost in terms of time and money of physically moving from the origin to the destination. Third, new migrants must familiarize themselves with local labor- and housing-market institutions once they arrive; they must find housing and work, and they must often adapt to a new culture or language. Empirical studies show that during the Great Migration, information was passed through letters that were often read by dozens of people and through conversation when migrants made trips back to their home communities. Thus early migrants provided information about labor- and housing-market conditions to friends and relatives who had not yet made the trip. First-time African American migrants often traveled with earlier migrants returning to the North after a visit to the South, which reduced physical costs. Additionally, previous migrants reduced new migrants * cost of adapting to a new locale and culture by providing them with temporary housing, food, and even credit. Previous migrants also provided a cultural cushion for later migrants, so that they did not have to struggle as hard with then- new surroundings.Q: The third and fourth paragraphs of the passage function primarily to Answer Choices: (A)cast doubt upon a historical explanation presented in the first paragraph (B)survey the repercussions of a historical event described in the first two paragraphs (C)derive a historical model from evidence presented in the first two paragraphs (D)answer a question raised in the second paragraph about a historical event (E)provide additional evidence for historical claims made in the first paragraph
A: Among A through E, the answer is
(A)cast doubt upon a historical explanation presented in the first paragraph
(B)survey the repercussions of a historical event described in the first two paragraphs
(C)derive a historical model from evidence presented in the first two paragraphs
(D)answer a question raised in the second paragraph about a historical event
(E)provide additional evidence for historical claims made in the first paragraph