If $\cos^{-1} x- \cos^{-1} \frac{y}{2} = \alpha$, then $4x^2 - 4xy \cos \alpha + y^2$ is equal to :

Question

( A ) $2 \sin 2 \alpha$
( B ) $4 \sin^2 \alpha$
( C ) $-4 \sin^2 \alpha$
( D ) $4$