under what condition(s) is the angular momentum of a rotating body, such as a spinning ice skater, conserved? (a) each external force acting on the body must be zero. (b) each external force and each external torque acting on the body must be zero. (c) each external force may be nonzero, but the sum of the forces must be zero. (d) each external torque may be nonzero, but the sum of the torques must be zero.