Defining options in a drop down list

Hi,
I would like to drive the available options in one list based on the entry in a previous list. So for example:

Column A options-
Dept a
Dept b
Dept c

Column B options:
if Dept a is selected:
Manager 1
Manager 2
Manager 3
If Dept B is selected
Manager 4
Manager 5
Manager 6
and so on...
I have all of my drop down lists set up, I'd just like to refine it one step further. Can anyone help please?

Thank you so much, this is perfect.