12. I have to see the doctor because I ______ a lot lately.

A. have been coughing B. had coughed       C. coughed               D. cough

分析：A。根據句中“不得不要去看醫生”這一信息可知，“咳嗽”這一現象現在還在持續，由此可排除B和C；比較A和D，顯然用現在完成進行時才能更好地表示從過去一直持續到現在的情況，故選A。

13. —Hi, Tracy, you look tired.

—I am tired. I ______ the living room all day.

A. painted                 B. had painted           C. have been painting D. have painted

分析：C。表示從過去某個時間開始一直延續到現在的動作，要用現在完成進行時。all day 是關鍵詞。

14. We ______ on this project for four hours. Let’s have a rest.

A. are working          B. have been working C. worked                D. had worked

分析：B。根據句中的let’s have a rest（我們休息一下吧）可知，連續工作四個小時導致我們現在很累了，所以用現在完成進行時比較合適。

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