题目内容：
Ⅱ. Vocabulary and Structure: (1*15=15 points,15%) Directions: There are 15 incomplete sentences or questions in this part. For each sentencethere are four choices marked A), B), C) and D). Choose the ONE that best completes the sentence. Then mark the corresponding letter on the Answer
Sheet. 21. A nuclear war would set()the development of all countries by centuries.
A. off
B. down
C. aside
D. back
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