I'd say that the correct answer is D, merely because all the other options don't seem correct to me. I wouldn't say A is correct because the book doesn't deal with Wall Street, but workers. B isn't correct as well, as the book doesn't focus on the cause of the drought, but its effects. C is also incorrect as people leave Dust Bowl, so in my opinion, the correct answer is the only remaining one - D. The book deals with the struggles of the workers, and their lives, and what they had to do to make for a living.
