Angular Displacement - 2011 A-level question

A disc rotates clockwise about its centre O until point P has moved to point Q, such that OP equals the length of the straight line PQ. What is the angular displacement of OQ relative to OP?

A.   \frac{\pi}{3} rad

B.   \frac{2\pi}{3} rad

C.   \frac{4\pi}{3} rad

D.   \frac{5\pi}{3} rad