23. A 5 × 5 square is divided into 25 cells. Initially all its cells are white, as shown on the left.
Neighbouring cells are those that share a common edge. On each move exactly two neighbouring cells
have their colours changed to the opposite colour (e.g. white cells become black and black ones become
white). What is the minimum number of moves required in order to obtain the chess-like colouring
shown on the right?
(A) 10 (B) 12 (C) 13 (D) 14 (E) 24
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