You are at a pizza joint that feature 15 toppings. You are interested in buying a 2- topping pizza. How many choices for the 2 toppings do you have in each situation below? (a) They must be two different toppings, and you must specify the order. (b) They must be two different toppings, but the order of those two is not important. (After all, a pizza with ham and extra cheese is the same as one with extra cheese and ham.) (c) The two toppings can be the same (they will just give you twice as much), and you must specify the order. (d) The two toppings can be the same, and the order is irrelevant.