private void timer1_Tick(object sender, EventArgs e)
 {
     ts          = ts - 1;
     lblmin.Text = Convert.ToString(ts / 60);
     lblsec.Text = Convert.ToString(ts % 60);
     if (ts == 0)
     {
         timer1.Enabled = false;
         rt2.Enabled    = false;
         my_messagebox m = new my_messagebox("Time Up!");
         m.ShowDialog();
         //MessageBox.Show("Time Up!");
         frm_result_session frm_res = new frm_result_session();
         frm_res.Show();
         this.Hide();
     }
 }