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（友情提示：鼠标拖选单词即时翻译）
21. I was halfway back to the hospital where the doctor was working______Susan caught up with me.
A. when B. while C. until D. though
22. —John, how did your English exam go？
—I thought I________have failed, but in fact I came in the top 10 in the class.
A. should B. couldn’t C. might D. mustn’t
23 --- When will you be through with your work, Jones?
---_______? My boss usually finds something for me to do at the last minute.
A. Would you help me B. Who knows C. I beg your pardon D. Anything the matter
24. Carbon dioxide____from burning fuels is the most common of the so – called greenhouse gases.
A. producing B. having been produced C. to be produced D. produced