need help in this probability and statistic question, with steps, thank for help

2. [10 points] A class of introductory probability consists of 10 sophomores, 60 juniors and 30 seniors. The final grades show that 5 sophomores, 9 juniors and 1 senior received an A grade. Assume B, C and D are the events that the student is a sophomore, junior and senior student, respectively. Let A be the event that the student got an A grade. (a) If a randomly chosen student is found to have earned an A grade, find the probability that he or she is a junior, that is P(CIA). (b) Find P(D) and P(DIA). (c) Are "getting an A grade" and "being a junior" independent events? How about the events of "getting an A grade" and "being a senior"