My late contribution:
The $6$ supplied by other answers could have been solved without filling in any possibles. There are three numbers $3,6,7$ missing from the centre column, and the $6$ can only go in the top cell.
Similarly there are three numbers $3,7,9$ missing from the second column, and the $3$ can only go in the cell at the top of the bottom block (yet you marked the possibilities $3,7$).