The integrating factor of differential equation $R \frac{dx}{dy} + Px = Q$ where $P$, $Q$, $R$ are functions of $y$ is (A) $e^{\int \frac{P}{Q}\,dy}$ (B) $e^{\int P\,dy}$ (C) $e^{\int \frac{P}{R}\,dy}$ (D) $e^{\int \frac{P}{R}\,dx}$