The following function computes XY for positive integers X and Y.int exp(int X, int Y) { int res = 1, a = X, b = Y; while ( b != 0 ){ if (b%2 == 0) {a = a*a; b = b/2; } else { res = res*a; b = b-1; } } return res;} Which one of the following conditions is TRUE before every iteration of the loop?

• A. XY = ab
• B. (res*a)Y = (res*X)b
• C. XY = res * ab
• D. XY = (res*a)b