in duty,on duty,off duty,duty free区别

题目

in duty,on duty,off duty,duty free区别
The students ________ should clean the classroom.
A.in duty
B.on duty
C.off duty
D.dutyl free
请问这四个选项的意思

答案

B
in duty有义务,有责任
as in duty bound
有责任,义不容辞
be in duty bound
有义务(做某事)
be in duty bound to do sth.
有义务(做某事)
on duty值班,当值
off duty下班
Only one doctor is on duty today - the other doctor is off duty.
今天只有一个医生值班,另一位下班了.
duty free免税