public ActionResult MarkExam()
        {
            if (Session["UserName"] == null)
            {
                return(RedirectToAction("LoginUser", "Login"));
            }
            Result rs   = new Result();
            Exam   exam = Session["Exam"] as Exam;

            if (!exam.Status)
            {
                return(RedirectToAction("Index", "ReSubmit_Error"));
            }
            exam.Status     = false;
            rs.Num_of_quizs = exam.Quizs.Count;
            exam.Mark_exam(db);
            rs.Num_of_correct_answers = exam.Count_correct_answers(db);
            rs.Score = Get_total_score(rs);
            Save_total_score_to_db(exam.Id_de_thi, rs.Score);
            return(RedirectToAction("Index", rs));
        }