Combobox's elements filtering
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 I have  combobox1 and comboox2 and in combobox1 my elements are A,B,C and combobox2       1,2,3,4,5,6...

       A related 1,2,3 and B related 3,4 and C related 5,6.
When I choose "A" I want to see just 1,2,3 ; when select "B" just 3,4 etc.
How can I imagine it ? 
I tried to do in selected index changed but I didn't 
        