In a puzzle of filling a grid, each row and column in the 9 × 9 grid, as well as each 3 × 3 sub-grid shown with heavy borders, must contain all the digits 1- 9. 

1	3		8			6
		2			7	A	B	C
			1	2		D	7	9
2	8
	9			3			1
							2	3
5	7			8	3
			4			9
		9			2		6	7

In the above partially filled grid, the number 3 appears in a square marked