(2013?Xuzhou second model) as shown in the figure, vertically upward in a uniform electric field, insulated lightweight spring stands vertically on the horizontal ground, a mass of m positively charge

(2013?Xuzhou second model) as shown in the figure, vertically upward in a uniform electric field, insulated lightweight spring stands vertically on the horizontal ground, a mass of m positively charged A, withdraw the force F, the ball from rest to leave the spring in the process, the ball by the upward elastic force, electric field force and downward gravity, because the electric field force to do positive work, so the mechanical energy increased, so A error;

B, the change in gravitational potential energy of the ball and gravity work is equal in size to the size of the work of the -W1, so B error;

C, the change in kinetic energy of the ball is equal to the work of the combined force, so C is correct;

C, the change in kinetic energy of the ball is equal to the work of the combined force. Therefore, C is correct;

D, the increase in the mechanical energy of the ball is equal to the work done by the rest of the forces except gravity, so the change in the mechanical energy of the ball is equal to the work done by the combined force of the electric field force and the spring elastic force, so D is incorrect;

Therefore, the choice of C.