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  • 科目:英语
  • 题型:单选题
  • 难度:中等
  • 人气:1285

According to Charles Keeling, the amount of carbon dioxide ________ sharply and steadily over the forty years between 1957 and 1997.

A.increased B.had increased
C.had been increased D.was increasing
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According to Charles Keeling,