In Sudoku, consider squares A1, A2... A9; B1, B2... B9; 11, 12... 19 where A, B ... I respectively denote the columns while 1, 2 ... 9 denote the rows; so A1 is column A, row 1; B2 is column B, row 2 etc. Given this, which of the following constraints would be required?
[Note: Sudoku is a game, where there is a 9x9 grid, with 9 mini-grids, each 3x3, from top left to bottom right, to be filled with numbers from 1 to 9 such that adjacent vertical cells must have distinct numbers, adjacent horizontal cells must have distinct numbers and each 3x3 grid must have distinct numbers]
A2 \neq B4 (A2 not equal to B4)
A3 \neq C5 (A3 not equal to C5)
A4 \neq D6 (A4 not equal to D6)
A1 \neq B2 (A1 not equal to B2)