$\cos^2 ( \frac{\pi}{6} + \theta) - \sin^2 (\frac{\pi}{6} - \theta)$ is equal to :

Question

( A ) $-\frac{1}{2}\cos 2 \theta$
( B ) $\frac{1}{2}$
( C ) $0$
( D ) $\frac{1}{2}\cos 2 \theta$