设 $\theta$ 是第二象限的角, 则必有
$\text{A.}$ $\operatorname{tg} \frac{\theta}{2}>\operatorname{ctg} \frac{\theta}{2}$
$\text{B.}$ $\operatorname{tg} \frac{\theta}{2} < \operatorname{ctg} \frac{\theta}{2}$
$\text{C.}$ $\sin \frac{\theta}{2}>\cos \frac{\theta}{2}$
$\text{D.}$ $\sin \frac{\theta}{2} < \cos \frac{\theta}{2}$