Solution: solution: a, b, let the gravitational forces of a and b be Ga and GB respectively. If Ga=Gbsinθ, the friction of B by C is zero; If Ga≠Gbsinθ, the friction between B and C is not zero; If GA < GBS in θ, the friction force of B subjected to C is upward along the slope; So, A is wrong and B is right.

C, D, taking the whole bc as the research object, the stress diagram is analyzed, and according to the equilibrium conditions, it can be known.

The friction force on the horizontal plane is f=Tcosθ=Gacosθ to c, and the direction is horizontal to the left.

In the process of sand flowing out slowly in A, the friction is decreasing, so D is right and C is wrong.

So choose: BD.