AMC 12 2004b Problem 19

A truncated cone has horizontal bases with radii $18$ and $2$. A sphere is tangent to the top, bottom, and lateral surface of the truncated cone. What is the radius of the sphere?

$\mathrm{(A)}\ 6 \mathrm{(B)}\ 4\sqrt{5} \mathrm{(C)}\ 9 \mathrm{(D)}\ 10 \mathrm{(E)}\ 6\sqrt{3}$