รบกวนสอบถามเรื่อง Dropdown หน่อยครับเกี่ยวกับการ ผูกข้อมูล 2 Dropdown

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
                    <ContentTemplate>
                            <asp:DropDownList ID="ddlProvince" runat="server" width="142px" 
                                DataSourceID="SqlDataSource1" DataTextField="PROVINCE_NAME_TH" 
                                DataValueField="PROVINCE_ID" 
                                onselectedindexchanged="ddlProvince_SelectedIndexChanged" 
                                AutoPostBack="True">
                            <asp:ListItem>-</asp:ListItem>
                            </asp:DropDownList>
                            <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
                                ConnectionString="<%$ ConnectionStrings:RMUTRConnectionString %>" 
                                SelectCommand="SELECT [PROVINCE_ID], [PROVINCE_NAME_TH] FROM [REF_PROVINCE]">
                            </asp:SqlDataSource>
                      </ContentTemplate>
                     </asp:UpdatePanel>
                                                                    <!--End Province-->

<asp:UpdatePanel ID="UpdatePanel2" runat="server">
                    <ContentTemplate>
                        <asp:DropDownList ID="ddlAmphur" runat="server" width="142px" 
                            AutoPostBack="True" 
                            onselectedindexchanged="ddlAmphur_SelectedIndexChanged" 
                          DataSourceID="SqlDataSource9" DataTextField="DISTRICT_NAME_TH" 
                            DataValueField="DISTRICT_ID">
                        <asp:ListItem>-</asp:ListItem>
                        </asp:DropDownList>
                       <asp:SqlDataSource ID="SqlDataSource9" runat="server" 
                            ConnectionString="<%$ ConnectionStrings:RMUTRConnectionString %>" 
                            SelectCommand="SELECT [DISTRICT_ID], [DISTRICT_NAME_TH] FROM [REF_DISTRICT]">
                        </asp:SqlDataSource>
                   </ContentTemplate>
                   </asp:UpdatePanel>

                                                                <!-- End Amphur-->

 protected void ddlProvince_SelectedIndexChanged(object sender, EventArgs e)
        {
            String strSQL;

            strSQL = "SELECT DISTRICT_ID,DISTRICT_NAME_TH FROM REF_DISTRICT WHERE PROVINCE_ID = '" + ddlProvince.SelectedValue + "' ";
            ddlAmphur.DataSource = clsDB.QueryDataTable(strSQL);
            ddlAmphur.DataTextField = "DISTRICT_NAME_TH";
            ddlAmphur.DataValueField = "DISTRICT_ID";
            ddlAmphur.DataBind();
           
           
        }

        protected void ddlAmphur_SelectedIndexChanged(object sender, EventArgs e)
        {
            String strSQL;

            strSQL = "SELECT DISTRICT_ID,DISTRICT_NAME_TH FROM REF_DISTRICT WHERE PROVINCE_ID = '" + ddlAmphur.SelectedValue + "' ";
            ddlAmphur.DataSource = clsDB.QueryDataTable(strSQL);
            ddlAmphur.DataTextField = "DISTRICT_NAME_TH";
            ddlAmphur.DataValueField = "DISTRICT_ID";
            ddlAmphur.DataBind();
        }