If $f(x)$ is integrable on $(0,a)$ then $\begin{align*} \int_0^a \frac{f(x)}{(x) + f(a-x) } dx \end{align*}$ is equal to :

Question

( A ) $0$
( B ) $\frac{a}{2}$
( C ) $a$
( D ) $1$