A semi-circle of radius 8\:cm, rocks back and forth along a line. The distance between the line on which the semicircle sits and the line above is 12\:cm. As it rocks without slipping, the semi-circle touches the line above at two points. (When the semi-circle hits the line above, it immediately rocks back in the other direction.) The distance between these two points, in millimetres, is closest to
Answer Choices
A. 55
B. 53
C. 51
D. 49
E. 47