Selecting the right combination to fill DropDown list in asp

I'm working on an admission from . I was struck in simple problem. I've fourDropDown lists.three to Select the Major subjects and one to show the list whom Candidate can apply. I want to restrict the user to apply for the specific subject on the basis of selected subject. I only want to show the only the specific subject in fourth dropDown list. Suppose if user Select Physics , Computer and Statistics . He is should not able to apply for the Computer Sci ,Bio, Mathematics etc. How can I restrict the candidates to apply on the basis of selected subjects