Excel 2007 : Eliminate choices from dropdown, based on cascading dependency

[lpellica]

I have a spreadsheet which has list boxes for each record (row) as follows:

• Section
• Role (dependent on Section selected)
• Class 1 (dependent on Role selected)
• Class 2 (same list dependent on Role)
• Class 3 (same list dependent on Role)
• Class ...25 (same list dependent on Role)

How would I make the Class dropdowns elminate what was selected in earlier Class selections (i.e., Class 3's dropdown would show the list of classes for that role, minus what was selected for Class 2 and what was selected for Class 1).

See attached sample file.