It is necessary that a college student __________ a foreign language.
A. should master
B. be mastered
C. mastered
D. should mastering
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He took his raincoat with him in case it __________.
A. rain
B. should rain
C. rains
D. had been rained
He is not a pilot, but he wishes he ____(be).
He ordered the work ____(start) at once.
清洗喷油器的作用是什么？
A. 防止喷油嘴积碳
B. 防止喷油堵塞
C. 雾化效果更好
D. 动力更强劲
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