As shown in the diagram, one end of a 11-foot-long electric cable is fixed at point A on the wall, and the other end of the cable (denoted by C ) is a sensor. There is a pin attached to the cable at a point B that is 4 units away from C, and the pin can be fixed to any point on the wall reachable by B. Due to gravity, the free part of the cable (i.e. BC ) is always perpendicular to the ground OD. A few other measurements are provided in the diagram. What is the minimum distance between C and D?