The second paragraph argues that initial drivers of the Great Migration are well understood but it is less clear why this migration continued, and even accelerated, in subsequent decades. Paragraphs 3 and 4 discuss the reasons for migration and provide specific examples of how migration became easier in subsequent decades due to reduced physical costs, reduced costs of adapting to a new locale and culture etc, thus answering the question posed in paragraph two - why the migration continued (D). Paragraph one only involves the discussion of initial drivers that prompted the Great Migration from South to North, whereas paragraphs 3 and 4 are concerned with the evidence supporting the continued migration in subsequent decades contrary to what (E) suggests.
