Suggest three classes, each consisting of n students. From this group of 3n students, a group of 3 students is to be selected.
a. How many choices are possible?
b. How many choices are there in which all 3 students are in the same class?
c. How many choices are there in which 2 of the 3 students are in the same class and the other student is in a different class.
d. How many choices are there in which all 3 students are in different classes?