Football team consists of 20 each freshman and sophomores, 15 juniors, and 10 seniors. Four players are selected at random to serve as captains. Find the probability that.
a. All 4 are seniors.
b. There is 1 each: freshman, sophomore, junior, and senior.
c. There are 2 sophomores and 2 freshmen.
d. At least 1 of the students is a senior.