Everything else covered: one mark, which could be either.
Two pictures in one
B or 13?
Read across, then read down. What is the mark in the middle?
The same mark: across, between A and C, it’s a B; down, between 12 and 14, it’s 13.
Why? Your brain reads with what it expects. Next to letters it sees a letter, and next to numbers a number. Jerome Bruner and Leigh Minturn showed this in 1955.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)