Hidden Single

If a cell is the only one in a row, column or block that can take a particular value, then it must have that value. All rows, columns and blocks, must contain each of the digits 1 to 9, so if there is only one cell that can hold a particular value, then it must hold that value.

For example, in the partial Sudoku puzzle below, the asterisk-marked cell is the only one in the center block that can hold a 2, and so it must hold a 2.

So why is this technique called hidden single? Simply because if you mark all the possibilities for every cell, these cells are the only ones to have a certain digit inside one or more of their containers, but they're hidden amongst the other candidates for the cell.

The following intermediate puzzle (from my interactive collection) contains one naked single (hilighted in pink), and five hidden singles, two of which I've highlighted in green. What makes them hidden singles? Can you spot the three other hidden singles?