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, 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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