Which of the following statements regarding the expansion of (x+y)^n are correct?
A. The coefficients of x^a y^b and x^b y^a are equal.
B. The coefficients of x^n-1 and y^n-1 both equal 1.
C. The coefficients of x^n and y^n both equal 1.
D. For any term x^a y^b in the expansion, a - b = n.