23. As shown in Figure 4, in the right triangle $\triangle ABC$, it is known that $\angle ABC = 90^{\circ}$. Semi-circles are constructed outward on each side of $\triangle ABC$. The area of the semi-circle with diameter $AB$ is $8\pi$, and the arc length of the semi-circle with diameter $AC$ is $8.5\pi$. Then the radius of the semi-circle with diameter $BC$ is ( ).
(A) 7
(B) 7.5
(C) 8
(D) 8.5
(E) 9
23. B.

From the problem, we know that $A B=8, A C=17$.
By the Pythagorean theorem, we have
$$
B C=\sqrt{A C^{2}-A B^{2}}=\sqrt{289-64}=15 \text {. }
$$

Thus, the radius of the semicircle with $B C$ as its diameter is 7.5.