问答题
if in case unless until when
问答题
We are unable to supply the goods ______ we receive payment in advance.
问答题
A spare axle is provided ______ one is damaged during routine use.
问答题
The machine should not be modified ______ a service engineer is present.
问答题
The filter should be changed ______ the unit has been in operation for two months.
问答题
The red light will come on ______ the machine overheats.
问答题
The machine should not be touched ______ it has cooled down.
问答题
There's a first aid box ______ someone hurts themselves.
问答题
The red light will not go out ______ the green switch has been pressed.
【正确答案】until or unless
【答案解析】