15 positive charges of q are distributed uniformly over the circumference of a circle with radius a. Then another charge of +Q is placed at the center of the circle. Which of the followings are true for this system? 1. The force acting on +Q is zero. II. If +Q is displaced perpendicular to the surface by a +Q small amount and released, it does not come back to the center. II. The electrostatic potential energy of +Q is IV. The magnitude of the electric field on +Q is 159Q III and IV IV O I, II, and I II Il and III O I, and II O I, II, and IV O I and II O I, II, II, and IV None of them O I and IV O II, III, and IV O Il and IV