A simple pendulum consists of sphere tied to the~end of a string of negligible mass. The sphere is~pulled back until the string is horizontal and then~released from rest. Assume gravitational potential~energy is zero when the sphere is at its lowest~point. What angle will the string make with the horizontal when the kinetic energy and the~potential energy of the sphere-Earth system is equal? 
选项为:A:Greater than 45°  B:Equal to 45°~  C:Less than 45°  D:It cannot be determined without knowing the mass of the sphere.  请回答其中一个选项.