The integrating factor of the differential equation $(1 - y^2) \frac{dx}{dy} + yx = ay$, $(-1 < y < 1)$ is (A) $\frac{1}{y^2 - 1}$ (B) $\frac{1}{\sqrt{y^2 - 1}}$ (C) $\frac{1}{1 - y^2}$ (D) $\frac{1}{\sqrt{1 - y^2}}$