C# ASP.Net - How to Display TextBox Based on Selected Value in DropDownList

<asp:DropDownList ID="ddlOptions" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlOptions_SelectedIndexChanged">
    <asp:ListItem Text="选项1" Value="1"></asp:ListItem>
    <asp:ListItem Text="选项2" Value="2"></asp:ListItem>
    <asp:ListItem Text="选项3" Value="3"></asp:ListItem>
</asp:DropDownList>

<asp:TextBox ID="txtValue" runat="server" Visible="false"></asp:TextBox>

protected void ddlOptions_SelectedIndexChanged(object sender, EventArgs e)
{
    if (ddlOptions.SelectedValue == "1")
    {
        txtValue.Visible = true;
    }
    else
    {
        txtValue.Visible = false;
    }
}