If three unit vectors $\overrightarrow {a},\overrightarrow {b},\overrightarrow {c}$ satisfy $\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c}=\overrightarrow {0}$ then the angle between $\overrightarrow {a}$ and $\overrightarrow {b}$ is :

Question

$$\begin {array} {1 1} (1)\;\frac{2 \pi}{3} & \quad (2)\;\frac{5 \pi}{6} \\ (3)\;\frac{\pi}{3} & \quad (4)\;\frac{\pi}{6} \end {array}$$