A ball of mass M moves in a circular path on a horizontal, frictionless surface. It is attached to a light string that passes through a hole in the center of the table. If the string is pulled down, thereby reducing the radius of the path of the ball, the speed of the ball is observed to increase. Complete the following sentence: This occurs because (a) the linear momentum of the ball is conserved. (b) it is required by Newton's first law of motion. (c) the angular momentum of the ball is conserved. (d) the angular momentum of the ball must increase. (e) the total mechanical energy of the ball must remain constant