TOEFL Reading: ETS-TOEFL阅读机经 - N6D3H6W7K5576UNGL$

According to paragraph 2 and 3, which of the following is one way in which Earth's early atmosphere differed from its current atmosphere? A. It had fewer greenhouse gases. B. It was warmer. C. It had lower levels of carbon dioxide. D. It did not contain methane.