# Let $\overrightarrow{a}, \; \overrightarrow{b}$ and $\overrightarrow{c}$ be three unit vectors such that $\overrightarrow{a} \times (\overrightarrow{b} \times \overrightarrow{c} ) = \frac{\sqrt 3}{2} (\overrightarrow{b} + \overrightarrow{c})$. If $\overrightarrow{b}$ is not parallel to $\overrightarrow{c}$, then the angle between $\overrightarrow{a}$ and $\overrightarrow{b}$ is :
( A ) $\frac{2 \pi}{3}$
( B ) $\frac{5 \pi}{6}$
( C ) $\frac{\pi}{2}$
( D ) $\frac{3 \pi}{4}$