code for combo box

Question

i have two combo box1 and combo box 2. cb1 contain I - year., II -  year., III - year... cb2 contains the first.,second.,third.,forth.,fifth.,sixth.,, now  i have to split each two items given to I - year... respectively..

i.e     I - year  ( first & second).. II - year (  third & forth ).. III - year ( fifth & sixth ).. 

now.. i have to select I - year in cb1 means, first and second items only visible on cb2.. 

help me ... code for that in c#.net