12. Let $0 < a < \Pi / 2$ be a fixed angle. If $P = ( \cos \theta , \sin \theta )$ and $Q = ( \cos ( a - \theta ) , \sin ( a - \theta ) )$ then Q is obtained from P by\\
(A) Clockwise rotation around origin through an angle a\\
(B) Anticlockwise rotation around origin through an angle a\\
(C) Reflection in the line through origin with slope tan a\\
(D) Reflection in the line through origin with slope $\tan \mathrm { a } / 2$\\