18. A circle with an area of 20 has its center at the fixed point as shown in Figure 4. $\triangle A B C$ is inscribed in this circle, and $S_{\triangle B B C}=8$, $\angle \alpha, \angle \beta, \angle \gamma$ are as shown in the figure. Then the value of $\sin \alpha+\sin \beta+\sin \gamma$ is ( ).
(A) $\frac{4 \pi}{5}$
(B) $\frac{3 \pi}{4}$
(C) $\frac{2 \pi}{3}$
(D) $\frac{\pi}{2}$
(E) $\frac{\pi}{4}$
18. A.

Let the center of the circle be $O$, and the radius be $r$. From the given conditions, we easily get
$$
\begin{array}{l}
S_{\triangle A B C}=\frac{1}{2} \cdot r^{2} \sin \alpha+\frac{1}{2} r^{2} \sin \beta+\frac{1}{2} r^{2} \sin \gamma=8 \\
\Rightarrow \sin \alpha+\sin \beta+\sin \gamma=\frac{16}{r^{2}} .
\end{array}
$$

The area of the circle is 20, i.e., $\pi r^{2}=20$, thus, $\frac{16 \pi}{20}=\frac{4 \pi}{5}$.