It definitely won't be B. I don't have the text with me, unfortunately, I'm sure it was a print-out; but judging by the title alone, I think we can infer that C and A were also not main ideas. Therefore, the answer would have to be D. Please correct me if I'm wrong, since you have the text with you. C is also a possibility. How often does the author mention C? Is it a point that they come back to multiple times? Is it mentioned in both the introduction, and the conclusion? All of these things point to main idea.