If $P = (0, 1, 2), \; Q = (4, -2,1), O = (0, 0, 0 )$, then $\angle{POQ}$ is equal to :

Question

( A ) $\frac{\pi}{2}$
( B ) $\frac{\pi}{3}$
( C ) $\frac{\pi}{6}$
( D ) $\frac{\pi}{4}$