A and B take the ball as the research object, and are in a state of balance under the action of gravity mg, slope supporting force N 1 and wall elasticity F, as shown in the right picture.

According to the equilibrium conditions: N 1 cosθ=mg, N 1sinθ=F, the solution is F=mgtanθ, so the static friction force is f=F=mgtanθ, N 1 = mgcosθ. When the inclination angle θ decreases, n 1 decreases and f decreases, which is determined by Newton's third law.

C and d choose the whole ball (M) and the inclined plane (M) as the research objects, which are in a state of balance under the action of gravity (M+m)g, ground support force N, wall elasticity F and ground static friction F, as shown in the left figure.

According to the equilibrium conditions: N-(M+m)g=0, f = F? When the inclination angle θ decreases, the static friction force F decreases and the inclined plane will remain stationary, so C is wrong and D is correct.

So choose AD.