Group the following figures into three classes on the basis of identical properties.
1
1, 8, 6; 2, 5, 7; 4, 3, 9
2
6, 1, 9; 7, 8, 5; 4, 2, 3
3
4, 2, 8; 7, 5, 9; 1, 3, 6
4
4, 2, 8; 7, 3, 6; 1, 5, 9
Group the following figures into three classes on the basis of identical properties.