Which of the following, if true, would provide a good reason for not trying to prevent cancer by the method indicated above? A. The human brain contains cell types other than neurons. B. The process of normal cell division has been studied extensively and is well understood. C. Most human tissue depends on the periodic division of cells for its health. D. The mechanism that keeps neurons from proliferating in adults has never been known to fail. E. Some kinds of cancer whose causes are known can be prevented now, but there are few kinds for which it is practically feasible to eliminate the causes.