Passage Summary: We found a specific mineral in 50-million-year-old lake deposits that can only be created if the air has a very high level of carbon dioxide.

Reasoning: Large nahcolite deposits formed in salty lakes during the Eocene epoch, and laboratory evidence shows this mineral only forms in salty water if atmospheric carbon dioxide exceeds 1,125 parts per million.

Analysis: This stimulus sets up a conditional relationship based on scientific evidence. We are told that 'Mineral Formation' requires 'High CO2.' Since we have factual evidence that the mineral did indeed form in those ancient lakes, we can safely conclude that the necessary condition—the high CO2 level—must have been present at that time. Look for an answer choice that simply connects these two dots to describe the Eocene atmosphere.