Derivative of $\cos^{-1} \left$ \frac{\sin x + \cos x}{\sqrt{2}} \right$$, $-\frac{\pi}{4} < x < \frac{\pi}{4}$ with respect to x is : (A) $-1$ (B) 1 (C) $\frac{\pi}{4}$ (D) $-\frac{\pi}{4}$