If $\vec{a}$ and $\vec{b}$ are two vectors such that $| \vec{a} | = 1, | \vec{b} | = 2$ and $\vec{a} \cdot \vec{b} = \sqrt{3}$, then the angle between $2\vec{a}$ and $-\vec{b}$ is : (A) $\frac{\pi}{6}$ (B) $\frac{\pi}{3}$ (C) $\frac{5\pi}{6}$ (D) $\frac{11\pi}{6}$