An examination is often regarded as good (i.e., has a valid grade spread) if the test scores of those taking it can be approximated by a normal distribution. The instructor uses the test scores to estimate parameters p. and :12. Then she assigns grades according to the followmg Eart: Students who score greater than ,u + cr .1 Students who soore between a and a + 11 Students who score between a w a and p \" Students who soore between p. _ 2a and ,u cr F Students who soore below - 2:1 - s This is sometimes called \"grading on the curve.\" (a) What is the probability that a given student will earn a A? (b) What is the probability that a given student will earn a D