The inclined plane is smooth. First, take A as the research object. According to the equilibrium condition: fA=mAgsinθ①, fA is upward along the slope, that is, the friction between C and A is fA, and the direction is upward along the slope.

According to Newton's third law, the friction between A and C is downward along an inclined plane. Taking c as the research object, the force analysis is carried out. According to the equilibrium condition: FB' = MCGS in FA+θ ②, FB' goes up along the inclined plane, which shows that FA < FB'= FA+θ② FA < FB.

According to Newton's third law, the direction of friction fB between C and B is downward along the inclined plane, that is, it is opposite to the direction of fA and greater than fA, so AD is wrong and BC is correct;

So: BC.