7. The marks for a university's entrance examination are known to be normally distributed wit ha mean of 70 and a variance of 144. The university is in the process of reviewing the grading criteria for the entrance examination. It wants to impose standards that will yield 20% A's, 22.9% B's, 35% C's, 14.1% D's and 8% F's. Where should the cutoff be: (a) between A's and B's? (b) between B's and C's