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重点讲解：
1. be on
（机器、电灯等）开着，工作着，使用中
eg. The light was on and the door was open.
灯亮着，门也敞开着。
eg. The television set was on.
电视机开着。
2. It's time to do sth.
是时候做某事；
eg. It's time to start swotting up for the exam.
现在该开始好好复习准备考试了。
eg. I think it's time to wind up this meeting.
我认为现在该结束这个会议了。
