If $\theta$ be the angle between two unit vectors. Evaluate $\sin \frac{\theta}{2}$

Question

$(A)\; \frac{1}{2} |\overrightarrow {a} - \overrightarrow {b}| = \sin \frac{\theta}{2}$
$(B)\; \frac{1}{2} |\overrightarrow {a} - \overrightarrow {b}| = \cos \frac{\theta}{2}$
$(C)\; \frac{1}{2} |\overrightarrow {a} - \overrightarrow {b}| = -\sin \frac{\theta}{2}$
$(D)\; \frac{1}{2} |\overrightarrow {a} - \overrightarrow {b}| = -\cos \frac{\theta}{2}$