Spent an entire class period going over this and teacher just laughed at us and refused to guide us in the right direction. We
The problem is that the student forgot about Newton's third law: if a first object exerts a force on a second, the second must exert a force of equal magnitude and opposite direction on the first. Thus all force coming from interactions between A and B, which define our system, must come in pairs. This is where the student failed. For example, P is a force exerted on A by B. Thus, in the free body diagram of B there must be a force exerted by A. This is C. Then $(P,C)$ are a pair related by Newton's third law. There is however a force in one of the diagrams that is unpaired in the other. Can you see which one it is?