I am having trouble with asp:UpdatePanel and Trigger of ASP.NET WebForms. Trigger could not find the event on my UpdatePanel. I've seen a lot of examples and I copied their implementations but could not get it right. I'm totally new in WebForms. Please help. Thank you.

```
                <tr>
                    <td class="label1">Will use ETL?</td>
                    <td>
                        <asp:UpdatePanel runat="server" UpdateMode="Conditional" ID="updatePanel1">
                            <ContentTemplate>
                                <asp:RadioButtonList ID="useEtl" runat="server" RepeatDirection="Horizontal" Width="120" OnSelectedIndexChanged="useEtl_SelectedIndexChanged">
                                    <asp:ListItem Text="Yes" Value="1" />
                                    <asp:ListItem Text="No" Value="0" />
                                </asp:RadioButtonList>
                            </ContentTemplate>
                        </asp:UpdatePanel>
                    </td>
                    <td class="label1">ETL Box</td>
                    <td>
                        <asp:UpdatePanel runat="server" UpdateMode="Conditional" ID="updatePanel2">
                            <ContentTemplate>
                                <asp:TextBox ID="etlBox" runat="server"/>
                            </ContentTemplate>
                            <Triggers>
                                 <asp:AsyncPostBackTrigger ControlID="useEtl" EventName="OnSelectedIndexChanged" />
                             </Triggers>
                        </asp:UpdatePanel>
                    </td>
                </tr>

```