A, C and C balls move in a straight line, and their forces are analyzed, as shown in the figure.

From Newton's second law, we get:

Mgsinθ=Ma ①

Zhang Liwei of thin line: T=Mgcosθ? ②

Then analyze the stress of the A ring, as shown in the figure below.

According to Newton's law, there are

mgsinθ-f=ma？ ③

N=mgcosθ+T ④

From the solution of ① ② ③ ④:

f=0

N=(M+m)gcosθ

Therefore, A is correct and C is correct;

Force analysis of B, D and D balls, due to gravity and tension, due to linear motion, the resultant force and velocity are in a straight line, so the resultant force is zero, the object moves at a uniform speed, and the tension of thin thread is equal to mg;

Then, the force of B is analyzed. As shown in the figure, due to gravity, tension and support, there must be backward friction, because of uniform motion, the resultant force is zero.

According to the equilibrium conditions, there are

(M+m)gsinθ=f

N=(M+m)cosθ

So, B is wrong and D is right;

So choose ACD. ..