difficulties in drop down list box

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ASP.NET

hi i had a table named as 'addcategory'
in that there 4 cols 'category,subject,question,answer'
in aspx form i have two drop down lists one for add category and other to ad subject
first i want to add category in dropdown1 and from that i select one category then add the related subjects in dropdown2 i also need there is no repeated values in both drop down lists ,also if one category is the particular subjects which are under that category should be in the subject drop down list wt can i do ? plz hlp me in detailed thanx a lot in advance