Block / Block
challenging
Firstly, if a number appears as candidates for only two cells in two different blocks, but both cells are in the same column or row, it is possible to remove that number as a candidate for other cells in that column or row.
For example, in the partial Sudoku puzzle below, the green cells marked with the asterisk are the only cells in their respective blocks that can contain a 3. This accounts for all the 3s in columns four and five. As there can be no other 3s in columns four or five, 3 can be eliminated as a candidate in the blue cells.