51. Agriculture is the country's chief source of wealth, wheat ___ by far the biggest cereal crop. A. isB. beenC. beD. being
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_______ the letter, he went out to post it. A. WritingB. Being writingC. Having writtenD. Written
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The officers narrowly escaped ______in the hot battle. A. have killed B. to killC. to be killedD. being killed
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Don't you remember _______ ? A. seeing the man beforeB. to see the man before C. saw the man beforeD. to have seen the man before
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We’re looking forward _____ the photo exhibition. A. to visitingB. to visitC. to having visited D. to visiting
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