Angle ABC of △ABC is a right angle. The sides of △ABC are the diameters of semicircles as shown. The area of the semicircle on AB equals 8π, and the arc of the semicircle on AC has length 8.5π. What is the radius of the semicircle on BC?
[asy]
import graph;
draw((0,8)..(-4,4)..(0,0)--(0,8));
draw((0,0)..(7.5,-7.5)..(15,0)--(0,0));
real theta = aTan(8/15);
draw(arc((15/2,4),17/2,-theta,180-theta));
draw((0,8)--(15,0));
label("A", (0,8), NW);
label("B", (0,0), SW);
label("C", (15,0), SE);[/asy]<spanclass=′latex−bold′>(A)</span>7<spanclass=′latex−bold′>(B)</span>7.5<spanclass=′latex−bold′>(C)</span>8<spanclass=′latex−bold′>(D)</span>8.5<spanclass=′latex−bold′>(E)</span>9