A passenger on a flatcar of a train moving at a constant horizontal velocity \(v_x\) throws a ball straight up relative to the train. An observer standing on the ground sees the ball follow a parabolic path. At the instant the ball reaches its maximum height relative to the ground, which of the following correctly describes the magnitude of the ball's acceleration, \(a\), and the rate at which the ball's speed is changing, \(\dfrac{dv}{dt}\)?