When the balls A and A reach the bottom of the vertical groove, they are pulled to the right by gravity and polished rod, and the resultant force inclines downward to the right, so does the acceleration. However, before reaching the bottom of the vertical groove, the acceleration of A is vertical, indicating that the acceleration of A is changing and it is impossible to accelerate uniformly. So, A is wrong.

At the moment when balls B, C and A reach the bottom of the vertical groove, the speed direction of A is vertical downward, so the speed of ball B is zero. The kinetic energy of ball B is zero first and then zero, so the kinetic energy of ball B increases first and then decreases, and the mechanical energy of the system is conserved, so the mechanical energy of ball A decreases first and then increases. Then the polished rod does negative work for ball A first, and then positive work for ball B, so it is wrong for B and correct for C;

When the mechanical energy of D.A. ball is minimum, the mechanical energy of B is maximum. This is an instantaneous state, which is the critical point where the polished rod does negative work and positive work on B, so the force is 0, so D is correct.

So choose CD.