题目

The hotel wasn't particularly good. But I ______ in many worse hotels. A．was staying             B．stayed              C．would stay       D．had stayed 答案：D4．设f（x，y，z）=sin2（x-y）+sin2（y-z）+sin2（z-x），x，y，z∈R，求f（x，y，z）的最大值．