题干
—— I can’t get through to the general manager’s office anyhow.
—— The line is busy. Someone _____ the telephone.
A：must use
B：use
C：must have been using
D：must be using
答案(点此获取答案解析）
D
是R上的偶函数，且
，则下列各式一定成立的
是( )
