Different combo boxes depending on parameter

Hi,

I have three materials. Each with their own sub types

Material - Type

A             - A1, A2, A3, A4

B             - B1, B2, B3, B4

C             - C1, C2

My idea is to let the user to select the material and type. I was thinking on having a combo box to select a material and then a second combo box that would change depending on the material selected.

I have been trying for quite a bit without luck. Maybe there is an alternative way of doing this? Any help is appreciated!

Regards,