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

A

$$2 \sin 2\alpha$$

B

$$4$$

C

$$4 \sin^2 \alpha$$

D

$$-4 \sin^2 \alpha$$