三级联动代码省级市级都能出来可是区级不出来是为什么？有大神帮我解答一下吗

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using LoginAndRegist.DAL;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

namespace LoginAndRegist
{
public partial class index : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (Session["HasLogin"] == null || Convert.ToBoolean(Session["HasLogin"]) == false)
{
Response.Redirect("Login.aspx");
}
else if (Convert.ToBoolean(Session["HasLogin"]) == true)
{
if (!IsPostBack)
{
DataTable dt = DBHelper.GetDataSet(string.Format("select * from AreaInfo where ParentID='1'"));

                ddlProvince.DataSource = dt;
                ddlProvince.DataTextField = "AreaName";
                ddlProvince.DataValueField = "AreaID";
                ddlProvince.DataBind();

                ListItem li = new ListItem("未选择", "0");
                ddlProvince.Items.Insert(0, li);
                ddlCity.Items.Insert(0, li);
                ddlCounty.Items.Insert(0, li);
            }
        }

    }

    protected void ddlProvince_SelectedIndexChanged(object sender, EventArgs e)
    {
        DataTable dt = DBHelper.GetDataSet(string.Format("select * from AreaInfo where ParentID='{0}'",ddlProvince.SelectedValue.ToString()));

        ddlCity.DataSource = dt;
        ddlCity.DataTextField = "AreaName";
        ddlCity.DataValueField = "AreaID";
        ddlCity.DataBind();

        ListItem li = new ListItem("未选择", "0");
        ddlCity.Items.Insert(0, li);

        ddlCounty.Items.Clear();
        ddlCounty.Items.Insert(0, li);
    }

    protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
    {
        DataTable dt = DBHelper.GetDataSet(string.Format("select * from AreaInfo where ParentID='{0}'",ddlCity.SelectedValue.ToString()));

        ddlCounty.DataSource = dt;
        ddlCounty.DataTextField = "AreaName";
        ddlCounty.DataValueField = "AreaID";
        ddlCounty.DataBind();

        ListItem li = new ListItem("未选择", "0");
        ddlCounty.Items.Insert(0, li);
    }
}

}

你调试下看下区有没有数据