A teacher distributes 10 identical homework passes to her class of 20 students. How many ways are there for her to distribute the passes if: (a) Each student gets at most one homework pass. (b) There are no restrictions on the number of homework passes a student can get. (c) There are no restrictions on the number of homework passes a student can get, except that one particular student, Sam, gets at least two homework passes. (d) There are no restrictions on the number of homework passes a student can get, except that one particular student, Sam, gets at most two homework passes.