回答：
"P" is for "permutation". A permutation is an ordered sequence of elements selected from a given finite set, without repetitions, and not necessarily using all elements of the given set.
P(n, r) = n!/(n-r)!.
"C" is for "combination". A combination is an un-ordered collection of distinct elements, usually of a prescribed size and taken from a given set.
C(n, r) = n!/[r!(n-r)!].