级联下拉列表更新问题ASP.NET

我有级联下拉列表，我希望第一个ddl触发一个函数并加载第二个ddl，第二个ddl触发另一个函数并加载第三个ddl

当我选择第一个ddl时，第二个ddl加载ok，但当我选择第二个ddl时，它仍然调用CategoryList1_SelectedIndexChanged方法，并且它们都调用相同的方法

我想不出是什么导致了这个问题。我也试过同步，但也不起作用

任何帮助都将不胜感激 干杯

KATEGORI:

MARKA:

型号：

<ajax:UpdatePanel ID="CategoryPanel" runat="server" UpdateMode="Conditional">
    <ContentTemplate>
    <div class="select">
    <ul>
       <li><strong>KATEGORI:</strong> 
        <asp:DropDownList ID="CategoryList1" AutoPostBack="true"  OnSelectedIndexChanged="CategoryList1_SelectedIndexChanged" runat="server">
        </asp:DropDownList>
        </li>
        <li><strong>MARKA:</strong> 
        <asp:DropDownList ID="CategoryList2" AutoPostBack="true"  OnSelectedIndexChanged="CategoryList2_SelectedIndexChanged" runat="server">
        </asp:DropDownList>
        </li>
        <li><strong>MODEL:</strong>
        <asp:DropDownList ID="CategoryList3" AutoPostBack="true"  OnSelectedIndexChanged="CategoryList3_SelectedIndexChanged" runat="server">
        </asp:DropDownList>
        </li>
        <li class="button"><asp:Button ID="GoButton" OnClick="GoButton_Click" Text="" runat="server" /></li>
        </ul>
    </div>
    </ContentTemplate>

     <Triggers>
            <asp:AsyncPostBackTrigger ControlID="CategoryList1" EventName="SelectedIndexChanged" />
            <asp:AsyncPostBackTrigger ControlID="CategoryList2" EventName="SelectedIndexChanged" />
            <asp:AsyncPostBackTrigger ControlID="CategoryList3" EventName="SelectedIndexChanged" />
    </Triggers>
</ajax:UpdatePanel>