How to Insert the selected dropdowns only.....

Hi....
Dear Friends......
I have small problem, i am using asp.net with vb.net (2005)
In my form has 2 text boxes 5 dropdowns, but i am not select all dropdons depending on the clien.... reqirments...( some times 2 or 3 or 4 or 5)
how can i insert the selected dropdowns only reming null values?
How can i over come this problem.... pls Help me (give the hint)
i write query string likt below ( i am sending the selected text only...)
insert into tablename (sno, name, ddl1, ddl2,ddl3, ddl4, ddl5)values('" & txtsno.text & "','" & txtname.text & "','" & ddl1.selecteditem.text & "','" & ddl2.selecteditem.text & "','" & ddl3.selecteditem.text & "','" & ddl4.selecteditem.text &am ...