大虾救命! dropdownlist三级联动实现不了?(VS2005+sql)

html
<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>无标题页</title>
    <script language="javascript" type="text/javascript">...
    //绑定City
    function BindToCity(ProvinceID,DDL_City_ID)
    {
        var DataSet_City = _Default.BindToCity(ProvinceID).value;//注意加Value
        var DDL_City = document.getElementById(DDL_City_ID);
        DDL_City.length = 0;//清除已有选项
        var Length = DataSet_City.Tables[0].Rows.length;
        if(Length!="0")
        {
            for(var i=0;i<Length;i++)
            {
                var Text = DataSet_City.Tables[0].Rows[i].Text;//必须和DataSet的列名一致
                var Value = DataSet_City.Tables[0].Rows[i].Value;
                DDL_City.options.add(new Option(Text,Value));
            }
        }
        else
        {
            var DDL_Area = document.getElementById("Area");
            DDL_Area.length = 0;
        }    }
    
    //绑定Area
    function BindToArea(CityID,DDL_Area_ID)
    {
        var DataSet_Area = _Default.BindToArea(CityID).value;
        var DDL_Area = document.getElementById(DDL_Area_ID);
        DDL_Area.length = 0;
        var Length = DataSet_Area.Tables[0].Rows.length;
        for(var i=0;i<Length;i++)
        {
            var Text = DataSet_Area.Tables[0].Rows[i].Text;
            var Value = DataSet_Area.Tables[0].Rows[i].Value;
            DDL_Area.options.add(new Option(Text,Value));
        }    }
 
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>请选择您所在的省：
        <asp:DropDownList ID="Province" runat="server">        
        </asp:DropDownList>
        
        请选择您所在的市：
        <asp:DropDownList ID="City" runat="server">        
        </asp:DropDownList>
        
        请选择您所在的区：
        <asp:DropDownList ID="Area" runat="server">
        </asp:DropDownList>
    </div>
    </form>
</body>
</html>cs:
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
        AjaxPro.Utility.RegisterTypeForAjax(typeof(_Default));
        this.BindToDDL();
    }    //获得DataSet方法
    private  DataSet GetDS(string SqlString)
    {
        SqlConnection con = new SqlConnection("server=.;database=ProvinceCityArea;uid=sa;pwd=;");
        SqlDataAdapter sda = new SqlDataAdapter();
        sda.SelectCommand = new SqlCommand(SqlString, con);
        DataSet ds = new DataSet();
        sda.Fill(ds);
        con.Close();
        con.Dispose();
        sda.Dispose();
        return ds;
    }
    
    //绑定Province，并添加事件。
    private void BindToDDL()
    {    
        string SQLSelect_Province = @"select * from province";
        this.Province.DataSource = GetDS(SQLSelect_Province);
        this.Province.DataTextField = "province";
        this.Province.DataValueField = "provinceID";
        this.Province.DataBind();
        this.Province.Items.Insert(0, "请选择");        this.Province.Attributes.Add("onchange", "BindToCity(this.options[selectedIndex].value,'City');");        this.City.Attributes.Add("onchange", "BindToArea(this.options[selectedIndex].value,'Area');");    }    //根据省ID查询相应的城市
    [AjaxPro.AjaxMethod]
    public DataSet BindToCity(string ProvinceID)
    {
        string SQLSelect_City = @"select city as Text,cityID as Value from city where father='" + ProvinceID + "'";
        return GetDS(SQLSelect_City);
    }    //根据城市ID查询相应的区域
    [AjaxPro.AjaxMethod]
    public DataSet BindToArea(string CityID)
    {
        string SQLSelect_Area = @"select area as Text,areaID as Value from area where father='"+CityID+"'";
        return GetDS(SQLSelect_Area);
    }
}web.config 里面也加了 <httpHandlers>
      <add verb="POST,GET" path="ajaxpro/*.ashx" type="AjaxPro.AjaxHandlerFactory, AjaxPro" />
    </httpHandlers>
我现在的问题是实现不了联动的. 选dropdownlist1, dropdownlist2还是空的.  onchange事件好像没有执行. 不知怎么回事
dropdownlist有onchange事件吗?