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： C8、已知函数f（x）=x3-sinx+1，若f（a）=3，则f（-a）=（　　）A、3B、-3C、-1D、-2