If $y = \frac{\cos x - \sin x}{\cos x + \sin x}$, then $\frac{dy}{dx}$ is : (a) $-\sec^2\left$\frac{\pi}{4} - x\right$$ (b) $\sec^2\left$\frac{\pi}{4} - x\right$$ (c) $\log \left| \sec \left$ \frac{\pi}{4} - x \right$ \right|$ (d) $-\log \left| \sec \left$ \frac{\pi}{4} - x \right$ \right|$