若 $\frac{\cos 2\alpha }{\sin (\alpha -{\displaystyle \frac{\pi }{4}})}=-\frac{\sqrt{2}}{2}$，则 $\cos \alpha +\sin \alpha$的值为

A．

$-\frac{\sqrt{7}}{2}$

B．

$-\frac{1}{2}$

C．

$\frac{1}{2}$

D．

$\frac{\sqrt{7}}{2}$