题目

Every possible means ______ to prevent the air pollution, but the sky is still not clear. A. is used    B. are used    C. has been used     D. have been used   答案：Cmean单数复数一个形式。在本题中代表单数。已知函数（其中ω为正常数，x∈R）的最小正周期为π．（1）求ω的值；（2）在△ABC中，若A＜B，且，求．