Let the vertices A and C of a right triangle ABC be on the arc with center B and radius 20. A semicircle with diameter [AB] is drawn to the inner region of the arc. The tangent from C to the semicircle touches the semicircle at D other than B. Let CD intersect the arc at F. What is ∣FD∣?<spanclass=′latex−bold′>(A)</span>1<spanclass=′latex−bold′>(B)</span>25<spanclass=′latex−bold′>(C)</span>3<spanclass=′latex−bold′>(D)</span>4<spanclass=′latex−bold′>(E)</span>5