Hide/Unhide Dropdownlist Items based on DropDownlList Items

Hi
I have two DropDownLists (DropDownList1,DropDownList2).i want when i select index1 of DropDownList1 then DropDownList2 first 2 index show and  3rd hide (Like this https://uogis.uog.edu.pk/DownloadNotes.aspx).....plzzz help me how can????.code is given below 
Asp.net <asp:DropDownList ID="DropDownList1" runat="server" Height="20px" Width="250px"> <asp:ListItem>Select</asp:ListItem> <asp:ListItem>A</asp:ListItem> <asp:ListItem>B</asp:ListItem> </asp:DropDownList> <asp:DropDownList ID="DropDownList1" runat="server" Height="20px" Width="250px"> <asp:ListItem>Select</asp:ListItem> <asp:ListItem>79.50%-84.49%</asp:ListItem> <asp:ListItem>69.50%-74.49%</asp:ListItem> </asp:DropDownList>