Complete Question Explanation
Must Be True. The correct answer choice is (D)
In this stimulus the author discusses the fact that large deposits of nahcolite, a rare mineral, were formed in salty lakes during the “Eocene epoch,” between 50 and 52 million years ago. In order for nahcolite to form in salty water, the atmosphere must have at least 1125 parts per million of carbon dioxide.
The stimulus is followed by a Must Be True question, so the correct choice must pass the Fact Test; the right answer must be confirmed by the information provided in the stimulus.
Answer choice (A): The author provides no basis for comparing the carbon dioxide levels in the atmosphere during the Eocene epoch with the levels since the Eocene epoch, so this choice cannot be confirmed by the information in the stimulus and it cannot be the right answer to this Must Be True question.
Answer choice (B): There is no mention of the degree of carbon dioxide fluctuation during the Eocene epoch, so this is not the right answer to this Must Be True question.
Answer choice (C): The lakes discussed were salty, and at some point during that period the atmosphere must have had at least 1125 parts per million of carbon dioxide (we know this based on the fact that nahcolite formed there during that period). This single instance does necessarily speak to a correlation between the saltiness of the lakes and the carbon dioxide levels of the atmosphere. This choice cannot be confirmed by the information in the stimulus, so it should be ruled out of contention.
Answer choice (D): This is the correct answer choice. Since the nahcolite discussed in the stimulus was formed in salty lakes, and the author provides that such formation requires a minimum of 1125 parts per million of carbon dioxide, this choice is confirmed by the information in the stimulus and must be the right answer to this Must Be True question.
Answer choice (E): The author’s discussion is limited to one instance of nahcolite formation, but that does not mean that no other such deposits have been found.