Passage Summary: A long time ago, the sun was much weaker than it is now. Even though a sun that weak should have turned the Earth into a giant ice cube, we have proof that there was liquid water and life on the planet billions of years ago.

Reasoning: The sun was significantly dimmer 2 billion years ago, which should have caused Earth's oceans to freeze, yet fossil evidence proves liquid water and life existed 3.8 billion years ago.

Analysis: We are faced with a contradiction between solar physics and biological history. If a dimmer sun should have resulted in a frozen planet, we need a reason why the Earth remained warm enough for liquid water and life to thrive. Look for an answer that introduces a warming mechanism, such as a different atmospheric composition or greenhouse effect, that was present in the past but is not present today. The correct choice will bridge the gap between the weak sun and the surprisingly warm Earth.