题目

 Scientists are convinced ________ the positive effect of laughter _______ physical and mental health. A.  of; at    B. by; in    C. of; on  D. on; at 答案：C两道单选题都含有A、B、C、D四个选项，瞎猜这两道题恰好全部猜对的概率为（ ）A．B．C．D．