Logical reasoning PrepTest 139 · Section 1 · Question 10

Incorrect. We have no information about the time period since the Eocene epoch. It could be true that CO2 levels dropped, but it could also be true that they haven't and nahcolite has continuously been able to form in salt water.

Incorrect. We know that atmospheric CO2 levels stayed above a certain level, but we can't tell if it fluctuated above that level.

Incorrect. There's no discussion of the likelihood of a lake being salty in any given time period, so this answer is out of scope.

Correct. Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
52-50m years ago - Large deposits of nahcolite formed in salty lakes
Nahcolite forms in salt water → Atmosphere has at least 1,125ppm CO2

Answer Anticipation:
This answer establishes a rule about the formation of nahcolite in salty water, and then it establishes that nahcolite formed in salty water during a certain time period. Since that formation meets the sufficient condition of the rule about nahcolite formation, we can conclude that in this time period (the Eocene epoch), the necessary condition must be true. In other words, there was at least 1,125ppm CO2 in the atmosphere during the Eocene epoch.

Answer Explanation:
This answer reflects the application of the rule about nahcolite formation to the Eocene epoch, when we know that some nahcolite formed in salt water. When a conditional ("only when") is applied to a specific case that meets the sufficient condition, we have a valid inference.

Key Takeaway:
When a Must Be True presents a single conditional, another statement will likely present information that allows it to be applied to a specific case. When this happens, the correct answer will apply the necessary condition to that specific case.

Incorrect. There's no discussion of more recent nahcolite formations or of any other time period, so this answer is out of scope.