急：使用ajax框架实现三级下拉菜单联动的问题

Question

使用ajax框架实现三级下拉菜单联动，但是改变第一个下拉菜单的时候，第二个下拉菜单会变化，第三个下拉菜单根本不变，这是怎么回事？
.aspx文件代码：
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
                                                                                    <ContentTemplate>
                                                                                        <table>
                                                                                            <tr>
                                                                                                <td>
                                                                                                    <asp:Label ID="Label4" runat="server">省：</asp:Label>
                                                                                                </td>
                                                                                                <td>
                                                                                                    <asp:DropDownList ID="ddlProv" runat="server" Width="100px" AutoPostBack="True" OnSelectedIndexChanged="ddlProv_SelectedIndexChanged">
                                                                                                    </asp:DropDownList>
                                                                                                </td>
                                                                                                <td>
                                                                                                    &nbsp; &nbsp; &nbsp;<asp:Label ID="Label5" runat="server">市：</asp:Label>
                                                                                                </td>
                                                                                                <td>
                                                                                                    <table>
                                                                                                        <tr>
                                                                                                            <td>
                                                                                                                <asp:DropDownList ID="ddlCity" runat="server" Width="100px" AutoPostBack="True" OnSelectedIndexChanged="ddlCity_SelectedIndexChanged">
                                                                                                                </asp:DropDownList>
                                                                                                            </td>
                                                                                                            <td>
                                                                                                                &nbsp; &nbsp; &nbsp;<asp:Label ID="Label6" runat="server">县(区)：</asp:Label>
                                                                                                            </td>
                                                                                                            <td>
                                                                                                                <asp:DropDownList ID="ddlCounty" runat="server" Width="100px" AutoPostBack="False">
                                                                                                                </asp:DropDownList>
                                                                                                            </td>
                                                                                                        </tr>
                                                                                                    </table>
                                                                                                </td>
                                                                                            </tr>
                                                                                        </table>
                                                                                    </ContentTemplate>
                                                                                    <Triggers>
                                                                                        <asp:AsyncPostBackTrigger ControlID="ddlProv" EventName="SelectedIndexChanged" />
                                                                                        <asp:AsyncPostBackTrigger ControlID="ddlCity" EventName="SelectedIndexChanged" />
                                                                                    </Triggers>
                                                                                </asp:UpdatePanel>

Answer 1

DEMO