A, taking the whole composed of three objects as the research object, supported by the total gravity and the ground A and B, the two supporting forces are equal. According to the equilibrium condition, the supporting force of the ground B is 2mg, and according to Newton's third law, the pressure of B on the ground is 2mg, so A is wrong.

B. There are two forces on the ground to A: supporting force and friction force, and the force on the ground to A is their resultant force; A is subjected to gravity mg, ground support force N 1, friction force f, and pressure N2 of C ball. As shown in the figure, according to the equilibrium condition, the resultant force of ground support force N 1 and friction force f is equal to and opposite to the resultant force of force mg and pressure N2, and the pressure N2 of C ball pair A is in AC direction, then the force mg and pressure.

The resultant force of N2 must not be along the AC direction, so the force on A is not along the AC direction, so B is wrong.

C, take C as the research object, and analyze the stress situation, as shown in the figure. The equilibrium conditions are as follows:

2 N2′cosθ= mg

N2 ′ = Mg2cosθ, and the smaller L and θ, the smaller the elastic force N2 ′ between A and C, so C is correct.

D. Taking A as the research object, according to the equilibrium conditions, it can be known that the friction force between the ground and A is f=N2sinα, while the pressure of C on A is N2 = N2' ′.

Then the smaller l is, the smaller α is and the smaller F. Therefore, D is wrong.

So choose C.