【题目】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
在线课程【答案】A
【解析】
试题分析：句中的时间状语over the forty years between 1957 and 1997是过去的时间状语，所以用一般过去时。故选A。