21. This time last week, we _______ for the exam. It was so hot in the examination room. A. are sitting B. were sitting C. had sat D. sat 22. The girls were chatting _______ the boys were playing games. A. when B. during C. while D. before 23. Nam _______ volunteer work when he _______ a university student. A. did / is B. did / was C. will do / was D. has done / is 24. My father _______ letters to my grandparents yesterday afternoon. A. write B. wrote C. has written D. is writing 25. While they _______ dinner, the telephone rang. A. had B. were having C. was having D. are having 26. My sister and I _______ a lot of clothes last summer. A. bought B. buy C. have bought D. buys 27. We _______ the keys while we were playing in the garden. A. lost B. were losing C. was losing D. have lost 28. My brother _______ English since 2003. A. learned B. have learned C. has learned D. is learning 29. Up to now, we _______ 3 CDs of My Tam. A. have bought B. bought C. has bought D. will buy 30. The man has been living here _______ 15 years. A. since B. for C. during D. at GIẢI THÍCH CHI TIẾT CHO EM VỚI Ạ EM CẢM ƠN