If $P(A \mid B) = P(A' \mid B)$, then which of the following statements is true ? (A) $P(A) = P(A')$ (B) $P(A) = 2 \, P(B)$ (C) $P(A \cap B) = \frac{1}{2} \, P(B)$ (D) $P(A \cap B) = 2 \, P(B)$