Mark against the correct answer in each of the following:

Mark $(\sqrt{)}$ against the correct answer in each of the following:

If $\boldsymbol{\theta}$ is the angle between two unit vectors $\hat{\mathrm{a}}$ and $\hat{\mathrm{b}}$ then $\frac{1}{2}|\hat{\mathrm{a}}-\hat{\mathrm{b}}|=$ ?

A. $\cos \frac{\theta}{2}$

B. $\sin \frac{\theta}{2}$

C. $\tan \frac{\theta}{2}$

D. none of these