retriving data from MS Acess2003 database

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C#3.0

C#

I have two combobox i.e combo1 & combo2 for selecting class and section respectively.
i want to show subjects of that particular class and section which are selected by the given two combobox.I am retriving subjects from Access2003 database.for i did the following code section combox_selectedindexchange ...

private void comboBox3_SelectedIndexChanged(object sender, EventArgs e)
        {
           String sub = ""; 
            try
            {
                conn.Open();
                OleDbCommand cmd = new OleDbCommand("select subject from ClassReg where Class_name='" + comboBox3.SelectedItem + "'  ", conn);
                OleDbDataReader rdr = cmd.ExecuteReader();
                while (rdr.Read())
                {

                  sub = Convert.ToString(rdr[0]);


                }
                
                if (sub == "")
                {
                    panel4.Hide();
                    MessageBox.Show("No subject is alocated for this class ");

                }
                else
                {
                    label8.Text = sub;

                    panel4.Show();
                }
                conn.Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
            }
       
        }

it shows only last class subject but i want to show all the subjects of that particular class and section.i am using c#

Edit: Code formatted