第三节：根据汉语意思补全句子。（每小题1分，共10分）
21. 很快，我就发现自己做的工作已经有人做过了，也就是说，我在浪费时间。
I soon found that the work I was doing had been done. ______ ______ ______, I was wasting time.
22. 大家都知道MUN是模拟联合国的简称。
Everyone knows MUN ______ ______ _____ Model United Nation.
23. 年底他们将离开这座城市。
______ ______ ______ of this year, they will leave the city.
