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Let $\vec a,\vec b\;$ and $\;\vec c$ be three unit vectors such that  $\vec a \times \left( {\vec b \times \vec c} \right) = \frac{{\sqrt 3 }}{2}\left( {\vec b + \vec c} \right)$ . If  $\vec b$ is not parallel to $\vec c$ then angle between $\vec a\;$ and $\;\vec b$ is :

• ✓
  $\frac{{5\pi }}{6}$
• B
  $\frac{{2\pi }}{3}$
• C
  $\frac{{3\pi }}{4}$
• D
  $\frac{\pi }{2}$