drop down list is not taking selected value

Question

Hello All,
 
           I am working on some project there i have 2 drop down lists and one Textbox in Update panel1. here i am binding the Names of PG from database to dropdownlist1(DDPGNames) and binding the PG rooms to dropdownlist2(DDRoomNumber) based first dropdownlist (it's like Country and state thing) now if i select any Room number in 2nd Dropdownlist the room rent has to dispaly automatically in textbox. for this i am writing this code on DDRoomNumber SelectIndexChanged Event. but my problem is DDRoomNumber is not taking selected room Number (It is taking Only First Value) if i select any other room in dropdown list it's going first one only.

see here my code:

    protected void DDRoomNumber_SelectedIndexChanged(object sender, EventArgs e)

        {

            txtRent.Enabled = true;

            MySqlConnection Con = new MySqlConnection(MyConString);

            MySqlCommand Cmd = Con.CreateCommand();

            string S = "SELECT*FROM pg_room_list WHERE Room_Num ='" + DDRoomNumber.SelectedItem.Text + "' AND Room_PG_ID ='" + DDPGNames.SelectedValue + "';";

            Cmd = new MySqlCommand(S, Con);

            MySqlDataReader Dr;

            Con.Open();

            Dr = Cmd.ExecuteReader();

            while (Dr.Read())

            {

                string SRoom_Rent = Dr.GetString("Room_Rent");

                txtRent.Text = SRoom_Rent;

            }

            Con.Close();    

        }          

Pls give me some solution.

Thanks