9. If $a \in \left(0, \frac{\pi}{2}\right)$, $\tan 2a = \frac{\cos a}{2 - \sin a}$, then $\tan a =$ A. $\frac{\sqrt{15}}{15}$ B. $\frac{\sqrt{5}}{5}$ C. $\frac{\sqrt{5}}{3}$ D. $\frac{\sqrt{15}}{3}$
9. If $a \in \left(0, \frac{\pi}{2}\right)$, $\tan 2a = \frac{\cos a}{2 - \sin a}$, then $\tan a =$\\
A. $\frac{\sqrt{15}}{15}$\\
B. $\frac{\sqrt{5}}{5}$\\
C. $\frac{\sqrt{5}}{3}$\\
D. $\frac{\sqrt{15}}{3}$