题目

The research group produced two reports based on the survey, but________ contained any useful suggestions. A . all B . none C . either D . neither 答案：D下列函数中，满足“?x1，x2∈（0，+∞），（x1-x2）[f（x1）-f（x2）]＜0”的是（　　）A．f（x）=lg（2x+1）B．f（x）=x+cosxC．f（x）=x2-1xD．f（x）=-x3-3x2