databound in dropdownlist

Question

i want to select any one value from dropdownlist1 according to that value, bound values in another dropdownlist2.

for ex,in dropdownlist1 there is class name like fy,sy,ty then according this if i choose fy class then in dropdownlist2 display the subjects in fy class but without click on any button. 

pls help me fast. It is very urgent for me.

Thank you...!