Which of the following data sets has no mode?

$the set 3 comma 3 comma 4 comma 5 comma 6$

$the set 1 comma 2 comma 3 comma 4 comma 5$

$the set 2 comma 2 comma 2 comma 3 comma 4$

$the set 8 comma 8 comma 9 comma 9 comma 9$