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(jù)句中“不得不要去看醫(yī)生”這一信息可知，“咳嗽”這一現(xiàn)象現(xiàn)在還在持續(xù)，由此可排除B和C；比較A和D，顯然用現(xiàn)在完成進(jìn)行時(shí)才能更好地表示從過(guò)去一直持續(xù)到現(xiàn)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(guò)去某個(gè)時(shí)間開始一直延續(xù)到現(xiàn)在的動(dòng)作，要用現(xiàn)在完成進(jìn)行時(shí)。all day 是關(guān)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(jù)句中的let’s have a rest（我們休息一下吧）可知，連續(xù)工作四個(gè)小時(shí)導(dǎo)致我們現(xiàn)在很累了，所以用現(xiàn)在完成進(jìn)行時(shí)比較合適。

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