18. As shown in Figure 6, rectangle $A B C D$ is inscribed in a semicircle, $E F$ is the diameter of the semicircle, $D A=16, F D=A E=9$. Then the area of rectangle $A B C D$ is $(\quad)$.
(A) 240
(B) 248
(C) 256
(D) 264
(E) 272
18. A.

Let the center of the semicircle be $O$, and its diameter be $9+16+9=34$.
Thus, $O C=17$.
By symmetry, $O$ is the midpoint of $D A$.
Then, $O D=O A=8$.
In the right triangle $\triangle O D C$, by the Pythagorean theorem, we have
$$
C D=\sqrt{17^{2}-8^{2}}=15 \text {. }
$$

Therefore, the area of rectangle $A B C D$ is $16 \times 15=240$.