The straight line AB is divided at C so that AC\equal{}3CB. Circles are described on AC and CB as diameters and a common tangent meets AB produced at D. Then BD equals:
<spanclass=′latex−bold′>(A)</span>diameter of the smaller circle<spanclass=′latex−bold′>(B)</span>radius of the smaller circle<spanclass=′latex−bold′>(C)</span>radius of the larger circle<spanclass=′latex−bold′>(D)</span>CB3<spanclass=′latex−bold′>(E)</span>the difference of the two radii