A college professor teaches Statistics 101 each fall to a large class of freshmen. For tests, he uses standardized exams that he knows from past experience produce bellshaped grade distributions with a mean of 70 and a standard deviation of 12. His philosophy of grading is to impose standards that will yield, in the long run, 20% A's, 26% B's, 38% C's, 12% D's, and 4% F's. Where should the cuto be between the A's and the B's? Between the B's and the C's?