Angle A B C of \triangle A B C is a right angle. The sides of \triangle A B C are the diameters of semicircles as shown. The area of the semicircle on \overline{A B} equals 8 \pi, and the arc of the semicircle on \overline{A C} has length 8.5 \pi. What is the radius of the semicircle on \overline{B C} ?
Answer Choices
A. 7
B. 7.5
C. 8
D. 8.5
E. 9