TOEFL Reading: ETS-TOEFL阅读机经 - 754THV75OVWCRQ65L$

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. A. Since the oceans and the atmosphere function together, the atmosphere must have become oxygen rich during this period. B. Because ocean water is in constant contact with the atmosphere, the two systems maintain a dynamic equilibrium. C. The transition to an oxygen-rich atmosphere could not have happened without constant contact with the oceans. D. Much of the oxygen in the oceans must have been pulled out of the atmosphere during this period.