The value of $\begin{align*} \int_{-\pi}^{\pi} \frac{\cos^2 x}{1+a^x} dx, a >0 \end{align*}$, is :

Question

( A ) $a \pi$
( B ) $\frac{\pi}{2}$
( C ) $2 \pi$
( D ) $\frac{\pi}{a}$