# If the matrix $\begin{bmatrix} \cos \theta & \sin \theta & 0 \\ \sin \theta & \cos \theta & 0 \\ 0 & 0 & 1 \end{bmatrix}$ is singular, then $\theta$ is equal to :
( A ) $\pi$
( B ) $\frac{\pi}{2}$
( C ) $\frac{\pi}{3}$
( D ) $\frac{\pi}{4}$