i need some answers for this question

<div class="fields">
                           <asp:DropDownList ID="ddreligion" runat="server" AutoPostBack="false" Width="150px">
                           </asp:DropDownList>
                       </div>
                       <div class="labels">
                           Caste
                       </div>
                       <div class="fields">

                               <asp:DropDownList ID="ddCast" runat="server">
                               </asp:DropDownList>


                       </div>

ddreligion loadin on page load event

using jquery ajax call i load cast second drop down list

$('#<%=ddreligion.ClientID%>').change(function () {
        $('#<%=ddCast.ClientID %>').empty().append($("<option></option>").html('Loading....'));
        $.ajax({
            type: "POST",
            url: 'Searchformatches.aspx' + '/populatecast',
            data: '{regionid: ' + $('#<%=ddreligion.ClientID%>').val() + '}',
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: OnDoctorsPopulated,
            failure: function (response) {
                alert(response.d);
            }
        });

    });

this is my web service
it is inside of my web form Searchformatches.aspx

[System.Web.Services.WebMethod]

        public static List<casts> populatecast(string regionid)
        {
            DataSet ds;
            SqlDataAdapter da;
            SqlConnection con;
           
            String constr = ConfigurationManager.ConnectionStrings["conn"].ConnectionString;
            con = new SqlConnection(constr);
            List<casts> cast = new List<casts>();

            da = new SqlDataAdapter("select castid,cast from dbo.castes where religionid =  " + regionid, con);
            ds = new DataSet();
            da.Fill(ds, "Cast");
            int i=0;
            casts c;
            for ( i=0;i<=ds.Tables [0].Rows .Count -1;i++)
            {
                c = new casts();
                c.castid=ds.Tables["Cast"].Rows[i][0].ToString();
                c.cast=ds.Tables["Cast"].Rows[i][1].ToString();
                cast.Add(c);
            }
             
            return cast;
        }</casts></casts></casts>

use selectedIndexchange event and bind child dropdown
or you can use jquery for same logic

put the drop down list in update panel
one by one in order write the server side events
<asp:updatepanel xmlns:asp="#unknown">
ddcountry load the daat in page load event

ddstate use where condtion ddcountry.selectedValue
ddcity use where condtion ddstate .selectedValue