4. ACT scores. The scores of students on the ACT college entrance examination in a recent year had a Normal distribution. with mean u = 18.6 and a standard deviation of o = 5.9. (a) What is the probability that a single student randomly chosen from all those taking the test scores 21 or higher? (b) Now take a simple random sample of 50 students who took the test. What are the mean and standard deviation of the sample mean score _x of these 50 students? (c) What is the probability that the mean score of these students is 21 or higher