Solution: Solution: First, take ball B as the research object, and analyze the stress situation. If you exert force as shown in figure 1, there will be

The pulling force of the thin wire on the ball B is T=mgcosθ(θ is the included angle between the thin wire and the vertical direction), and the pressure of the vertical rod on the ball B is N=mgtanθ.

When the ball A moves slowly to the right, θ increases and cosθ decreases, then T increases and N increases, and the pressure of the ball B on the vertical rod gradually increases.

Then take the whole as the research object, and analyze the stress as shown in Figure 2, then

The supporting force fn of the horizontal rod to the A ring is 2 mg. If it is unchanged, the sliding friction force f=μFN, and f remains unchanged.

F=N+f, if n increases, then f increases. So, AD is right and BC is wrong.

So choose AD.