Combobox which are dependent
 

I have 2 columns with related values in it. The number of items can
grow.

A 2
B 3
C 2
D 1

I have a form with 2 comboboxes. What I would like is that if the user
selects A in the first combobox, the second combobox is automatically
populated with 2. The same thing should happen with combobox2: If the
user selects 2, combobox 1 should only display A and C.

Thanks!!