$21 \cdot 6$ As shown in the figure, there are two ribbons of width 1, intersecting at an angle of $\alpha$, the area of the shaded part is
(A) $\sin \alpha$.
(B) $\frac{1}{\sin \alpha}$.
(C) $\frac{1}{1-\cos \alpha}$.
(D) $\frac{1}{\sin ^{2} \alpha}$.
(E) $\frac{1}{(1-\cos \alpha)^{2}}$.
[Solution] It is clear that the shaded part is a rhombus. Let its side length be $a$, then
$$
\begin{aligned}
& a=\frac{1}{\sin \alpha} . \text { (as shown in the figure) } \\
\therefore \quad & S_{\text {shaded }}=a \cdot 1=\frac{1}{\sin \alpha} .
\end{aligned}
$$

Therefore, the correct choice is $(B)$.