Construct a wiring diagram for a cell that responds to "A or B if not C." This is trickier than it sounds. If you simply shift the threshold of cell X to 1, it will respond to "A if not C, or B if not C, or A and B even if C." Can you get X to respond to either A or B, but only if C is inactive? (Hint: You might need to introduce more than just cells A, B, and C.)