The cell marked AB is the ‘hinge’. If the solution to that cell turns out to be A then C will definitely occur in the lower left corner. If AB turns out to be B then C is certain to occur in the top right. AC/BC is a *complimentary* pair, meaning they will both be either true or false.
So whatever happens, C is certain in one of those two cells marked AC or BC. The cell marked with a cross can be 'seen' by both Cs - the cell is a confluence of both BC and AC. It’s impossible for a C to live there and if C resides there it can be evicted. |