$\text{A.}$ $\sqrt{\frac{1-\cos 2 \alpha}{1+\cos 2 \alpha}}$
$\text{B.}$ $\sqrt{\frac{1+\cos (\pi+2 \alpha)}{2}} \cdot \frac{1}{\cos \alpha}, \alpha \in(0, \pi)$
$\text{C.}$ $\frac{1-\cos 2 \alpha}{\sin 2 \alpha}$
$\text{D.}$ $\frac{\sin 2 \alpha}{1-\cos 2 \alpha}$
$\text{E.}$
$\text{F.}$