We ______ careful when we are walking across the road. There are too many vehicles now. [ ]

A. can be

B. may be

C. must be

下列关于MTF的组合，错误的是（）

A．MTF——影像清晰度的评价

B．屏/片体系MTF——使用矩形波测试卡

C．MTF上升——空间频率高

D．焦点MTF测定——傅氏变换法

E．MTF上升——屏一片感度低