已知 $\tan \beta=\frac{\cos \alpha}{1-\sin \alpha}, \tan (\alpha+\beta)=\frac{1+\sin \alpha}{\cos \alpha}$ ，若 $\beta \in\left(0, \frac{\pi}{2}\right)$ ，则 $\beta=$
$\text{A.}$ $\frac{\pi}{12}$
$\text{B.}$ $\frac{\pi}{6}$
$\text{C.}$ $\frac{\pi}{4}$
$\text{D.}$ $\frac{\pi}{3}$
$\text{E.}$
$\text{F.}$