12. The condition for the function $f(x)=3 \sin (2 x+\theta)$ to be an even function is (
(A) $\theta=k \pi+\frac{\pi}{2}(k \in Z)$
(B) $\theta=k \pi+\pi(k \in Z)$
(C) $\theta=2 k \pi+\frac{\pi}{2}(k \in Z)$
(D) $\theta=2 k \pi+\pi(k \in Z)$
12. A