. a stone of mass m is thrown upward at a 30o angle to the horizontal. at the instant the stone reaches its highest point, why is the stone neither gaining nor losing speed? (pick one) a) because the acceleration of the stone at that instant is 0; b) because the net force acting upon the stone at that instant has magnitude mg; c) because the angle between the stone’s velocity and the net force exerted upon the stone is 90o; d) because the stone follows a parabolic trajectory and th peak of the trajectory is where the parabola has zero slope.