The crux of the combinatorial proof of Pascal's formula is that the set of subsets of size r of a set {x_1, x_2,..., x_n+1} can be partitioned into the set of subsets of size r that contain _______ and those that _______.
a. x_n+1, do not contain x_n+1
b. x_1, do not contain x_1
c. x_2, do not contain x_2
d. x_n, do not contain x_n