How to manipulate the contents of PopupMenu function?

I intend to have a first PopupMenu function with a list of parameters {a,b,c}. I want a second PopupMenu list of contents to change according to what's in the first list like so:

• if the first list has 'a' selected then the second list is {1,2,3},
• if the first list has b selected then the second list is {4,5,6},
• if the first list has 'c' selected then the second list is {7,8,9}.

I have tried the following:

Row[{
}]


but it is not interpreted the way I intended.

How can I accomplish it?

You forgot about Dynamic here and there.
Row[{