The integrating factor of the differential equation $(1-y^2)\large\frac{dx}{dy}$$+yx=ay\: \;is$

Question

\[(A)\;\frac{1}{y^2-1}\qquad(B)\;\frac{1}{{\sqrt{y^2-1}}}\qquad(C)\;\frac{1}{1-y^2}\qquad(D)\;\frac{1}{{\sqrt{1-y^2}}}\]