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Hi!

Who can help me!
I don't understand this: (marked with ???)

Code:

8 . 9 | 6 3 . | 7 . .
. . . | . . . | 8 . .
4 5 . | . . . | . 1 .
----------------------
. . . | 1 . . | . 7 3
. . . | 8 2 5 | 9 . 1
1 9 . | . . . | . 2 8
----------------------
. 3 . | . . . | . 9 6
. . 7 | . . . | . 8 .
. . . | . 1 4 | 2 . 7



OK The value 3 in Box [2,1] must lie in Row 5.
OK The value 4 in Box [1,3] must lie in Row 1.
OK The value 5 in Box [1,3] must lie in Row 1.
OK The value 5 in Column 7 must lie in Box [2,3].
OK The value 6 in Box [3,1] must lie in Row 9.
OK The value 7 in Box [1,2] must lie in Row 3.
OK The value 8 in Box [3,1] must lie in Row 9.
OK The values 4 and 5 occupy the cells (1,8) and (1,9) in some order.
OK The values 4 and 5 occupy the cells (2,4) and (2,5) in some order.
OK The values 3 and 6 occupy the cells (3,3) and (3,7) in some order.

??? The values 1 and 4 occupy the cells (7,3) and (7,7) in some order.
??? The values 3 and 7 occupy the cells (2,1) and (5,1) in some order.
??? The values 1 and 4 occupy the cells (7,3) and (8,2) in some order.
??? The value 2 in Column 1 must lie in Box [3,1].
??? The values 2 and 8 occupy the cells (4,2) and (4,3) in some order.

?? The values 1 and 4 occupy the cells (7,3) and (7,7) in some order.

This has exactly the same logic as the previous 3 lines that you marked as OK. The numbers 1 and 4 appear as candidates in only two cells in row 7. Therefore, one number must be in each cell, so any other candidates for those cells can be eliminated.

Similarly for the next few lines.