In "The Sun Game", two players take turns placing discs numbered 1 to 9 in the circles on the board. Each number can only be used once. The object of the game is to be the first to place a disc so that the sum of the 3 numbers along a line through the centre circle is 15. (a) If Avril places a 5 in the centre circle and then Bob places a 3, explain how Avril can win on her next turn.
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