问如何以编程方式启用下拉列表元素

EN

<asp:DropDownList ID="DropDownList9" runat="server" Width="128px"  onchange="display()"  >
        <asp:ListItem Value="yes">Yes</asp:ListItem>
        <asp:ListItem Value="no">No</asp:ListItem>
    </asp:DropDownList>

<asp:DropDownList ID="DropDownList10" runat="server" Width="107px" TargetControlID="DropDownList9" >
        <asp:ListItem Value="completed">Completed</asp:ListItem>
        <asp:ListItem Value="ongoing">Ongoing</asp:ListItem>
        <asp:ListItem Selected="True" Value="none" Enabled="False">[SELECT]</asp:ListItem>
    </asp:DropDownList>

<script type="text/javascript" language="javascript">
    function display()
     {
         if (document.getElementById('<%=DropDownList9.ClientID%>').value == "no") 
        {
            document.getElementById('d1').style.visibility = "hidden";
            document.getElementById('<%=DropDownList10.ClientID%>').style.visibility = "hidden";
            document.getElementById('<%=DropDownList10.ClientID%>').value = "none";
            //DropDownList10.SelectedValue = "none";   not wrkin
        }
        else {
            document.getElementById('<%=DropDownList10.ClientID%>').style.visibility = "visible";
            document.getElementById('d1').style.visibility = "visible";
        }
    }
 </script>