static void Game(ref Prisoner a, ref Prisoner b, int round)
        {              // ^我後來發現這邊寫法有點多此一舉,物件作為引數被讀入後,本來就是傳址,不是傳值。故不用ref
            switch (a.LastChoice)
            {
            case (Decision.COOPERATE):
                if (b.LastChoice == Decision.COOPERATE)
                {   // win-win
                    a.score += 3;
                    b.score += 3;
                }
                else
                {   // b win
                    b.score += 5;
                }
                break;

            case (Decision.BETRAY):
                if (b.LastChoice == Decision.COOPERATE)
                {   // a win
                    a.score += 5;
                }
                else
                {   // Both Betray
                    a.score += 1;
                    b.score += 1;
                }
                break;
            }
            ScoreBoard(ref a, ref b, round);
            Decision a_choice = a.LastChoice, b_choice = b.LastChoice;

            a.Action(b_choice);
            b.Action(a_choice);
        }// End of void Game