Combination/Permutation Problem

A television director is scheduling a certain sponsor's commercials for an upcoming broadcast. There are six slots available for commercials. In how many ways may the director schedule the commercials?

a) If the sponsor has 6 different commercials, each to be shown 1 time?

(This one I know is 6!)

b) If the sponsor has 3 different commercials, each to be shown 2 times?

From here onwards I don't know how to approach the question.

c) If the sponsor has 2 different commercials, each to be shown 3 times?

d) If the sponsor has 3 different commercials, the first of which is to be shown 3 times, the second 2 times and the third 1 time?

Can anyone please explain how to answer this question, thanks.(Headbang)