(C) is incorrect because while it is one of the points made in the passage, it is not the main point or the main argument the author is trying to make. For an answer to the main point question, try to ask yourself what is the purpose of this passage? What is the main idea that the author is trying to convey?
Here, the author makes an argument on lines 19-23 "less clear...why migration continued, and even accelerated, in subsequent decades.." The rest of the passage is used to attempt to answer this question as (E) suggests. Hence, we can conclude that the main purpose of the passage is to explain the factors that contributed to the duration and scope of the Great Migration.
