public void Display(out option_tinh_tnk op_option, out decimal op_so_tien, out decimal op_phan_tram_thue)
 {
     ShowDialog();
     op_option = _option;
     op_so_tien = _so_tien;
     op_phan_tram_thue = _phan_tram_thue;
 }
 private void lay_option()
 {
     if(radioButton1.Checked == true)
     {
         _option = option_tinh_tnk.TONG_TIEN;
         return;
     }
     if(radioButton2.Checked == true)
     {
         _option = option_tinh_tnk.DON_GIA;
         return;
     }
     if(radioButton3.Checked == true)
     {
         _option = option_tinh_tnk.KHONG_TINH;
         return;
     }
     _option = option_tinh_tnk.CHUA_CHON;
 }