You indicated that the acceleration at point D is opposite in direction to the velocity.  When the angle between the velocity and acceleration is 180°, then the ball slows down while moving in a straight line.  Is the ball moving in a straight line at point D?

In addition, remember that at point D the ball is flying through a vacuum.  Hence, the acceleration of the ball at point D is the freefall acceleration (i.e., the acceleration of gravity).  In what direction is the freefall acceleration?

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