PETER WANTS TO COLOUR THE CELLS OF A 3 × 3 SQUARE IN SUCH A WAY TH...

18. Peter wants to colour the cells of a 3 × 3 square in such a way that each of the rows,

the columns and both diagonals have three cells of three different colours. What is

the least number of colours Peter could use?

(A) 3 (B) 4 (C) 5 (D) 6 (E) 7