If P(0, 1, 2), Q(4, -2, 1) and O(0, 0, 0) are three points, then ∠POQ =

$\frac{\pi}{6}$ $\frac{\pi}{4}$ $\frac{\pi}{3}$ $\frac{\pi}{2}$

$\frac{\pi}{2}$

Directions ratios of OP and OQ are proportional to 0, 1, 2 and 4, -2, 1 respectively. $∴ cos∠POQ = \frac{0×4+1×(-2)+2×1}{\sqrt{0+1+4}\sqrt{16+4+1}}= 0 ⇒ ∠POQ = \frac{\pi}{2}$