If $\overrightarrow {a} + \overrightarrow {b}+ \overrightarrow c=\overrightarrow {0}$ and $| \overrightarrow {a} |=3, | \overrightarrow {b}|=4$ and $|\overrightarrow {c}|=\sqrt {37},$ then the angle between $\overrightarrow{a}$ and $\overrightarrow {b}$ is :

Question

$$\begin {array} {1 1} (a)\;\frac{\pi}{4} & \quad (b)\;\frac{\pi}{2}\\ (c)\;\frac{\pi}{6} & \quad (d)\;\frac{\pi}{3}\end {array}$$