A, B, C be events with Pr(C) > 0. Then Pr(A - B|C) =
a) Pr(A|C) - Pr(A ∩ B|C)
b) Pr(A|C) + Pr(B|C)
c) Pr(A|C) / Pr(B|C)
d) Pr(A|C) * Pr(B|C)