If $g(x)=f(x)+f(1-x)$ and $f^{\prime \prime}(x)<0$ for $0 \leq x \leq 1$, then
(A) $g(x)$ increases in $\left(-\infty, \frac{1}{2}\right)$
(B) $g(x)$ increases in $\left(0, \frac{1}{2}\right)$
(C) $g(x)$ decreases in $\left(\frac{1}{2}, 1\right)$
(D) $g(x)$ decreases in $\left(\frac{1}{2}, \infty\right)$