18. As shown in Figure 8, in the right triangle $\triangle ABC$, $\angle ABC = 90^{\circ}$, $AB = 8$, $BC = 6$. Circles are drawn with $A$ and $C$ as centers and $\frac{AC}{2}$ as the radius, cutting out two sectors from the right triangle $\triangle ABC$. The area of the remaining (shaded) part is ( ).
(A) $24 - \frac{25 \pi}{4}$
(B) $\frac{25 \pi}{4}$
(C) $24 - \frac{5 \pi}{4}$
(D) $24 - \frac{25 \pi}{6}$