TOEFL Reading: ETS-TOEFL阅读机经 - 03177I048M55091UL$

In paragraph 2, why does the author provide the information that methane and carbon dioxide kept the Earth warm during the Sun's early history? A. To explain how the early Earth and the early Sun were related B. To support the claim that methane and carbon dioxide are greenhouse gases C. To explain why the high levels of methane and carbon dioxide in Earth's early atmosphere were favorable for early life D. To suggest that these gases affect how brightly the Sun burns