ajax组合框不起作用

Question

I am trying to use ajax combobox in my asp.net web forms.

I have added ajax toolkit and I can use other controls like PopupControlExtender etc. but when I use comboBox it gives an error saying ComboBox is not a known element.

I right-clicked on AjaxControl Toolkit in the Toolbar--> Choose Items and verified that ComboBox is checked but it doesn't appear in the ToolBar

Does anyone know what the issue may be or has faced a similar problem ?

EDIT :

Code :

        <div>
            <asp:ScriptManager runat="server" ID ="ScriptManager1" />
                 Describe how spicy you like your food:
            <br />
            <cc1:ComboBox ID="ComboBox1" runat="server">
                <asp:ListItem Text="Mild" Value="0" />
                <asp:ListItem Text="Medium" Value="1" />
                <asp:ListItem Text="Hot" Value="2" />
            </cc1:ComboBox>     

           <%-- <asp:Button
                ID="btnSubmit"
                Text="Submit"
                Runat="server" OnClick="btnSubmit_Click" />--%>


        </div> 

Also,I have added this to the top of my page after the page directive :

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

Any idea,anyone ?