Combo Box Dependencies and Population

Hello,

I am new to this site and have perhaps a amateur question about combo boxes. What I am trying to do is make two types of combo boxes. The first combo box is independent and depending on what is selected, the dependent combo box generates a unique list based on the what choice was selected in the independent combo box.

To help, I have attached a pretty clear example of what I'm trying to do here.

Thanks in advance!