$\frac{d}{dx} \{ \cos^{-1} x + \sin^{-1} \sqrt{1-x^2} \}$ is equal to :

Question

( A ) $0$
( B ) $1$
( C ) $-\frac{2}{\sqrt{1-x^2}}$
( D ) $\frac{2}{\sqrt{1-x^2}}$