If you don't know a person's last name, use 'ma'am' or 'sir'.
如果你不知道一人的姓, 你应称呼他为 'ma'am' 或 'sir' 。
Inviting the fire brigade to put out a nonexistent fire is a crude form of deception which no self-respecting student would ever indulge in.
请消防队来扑灭一场根本没有的大火是一种低级骗局,有自尊心的大学生决不会去做。