If $\int f(x) \sin x \cos x d x=\frac{1}{2\left(b^{2}-a^{2}\right)} \log f(x)+c$
then $f(x)$ is equal to(A) $\frac{1}{a^{2} \sin ^{2} x-b^{2} \cos ^{2} x}$
(B) $\frac{1}{a^{2} \cos ^{2} x+b^{2} \sin ^{2} x}$
(C) $\frac{1}{a^{2} \sin ^{2} x+b^{2} \cos ^{2} x}$
(D) none of these