I'm not sure how to go about it? I think for part a, would be 16C5, or am I wrong?

Question

The outcome of chance events in a fantasy role-playing game is determined by rolling polyhedral dice with anywhere from 4 to 20 sides. Suppose you roll a 16 sided die 5 times and observe the number on the top of the die.
a) How many possible outcomes are there for these 5 rolls?
b) In how many of the outcomes in the part (a) do the 5 rolls produce five different numbers?
c) What is the probability that at least 2 of the rolls are the same?