Grades in large classes are sometimes approximately Normally distributed-a fact that serves as the justification for grading on a bell

Please help by explaining the concepts underlying how the following problem is solved

Grades in large classes are sometimes approximately Normally distributed-a fact that serves as the justification for "gradingon a bell curve." A common practice for very large classes is to give 16% of students A grades, 34% B grades, 34% C grades,and 16% D and F grades. Assuming a Normal distribution of grades, what are the z-scores for these letter grades?z> 10<z <1-1 <z<0z < -1Answer BankB gradesD and F gradesA gradesC grades