PrepTest 139, Section 1, Question 10
Large deposits of the rare mineral nahcolite formed(38 more words not shown)
The statements above, if true,(8 more words not shown)
- AFor most of(26 more words not shown)
- BLevels of carbon(10 more words not shown)
- CLakes were more(22 more words not shown)
- DThe atmosphere contained(18 more words not shown)
- ENo significant deposits(11 more words not shown)
Reading the argument
The mineral formed, so the conditions it requires must have been present.
Nahcolite formed in salty lakes during part of the Eocene epoch. The laboratory tests tell us that formation in salty water requires at least 1,125 parts per million of atmospheric carbon dioxide. Put those facts together: the atmosphere must have reached that threshold at some point during the period when those deposits formed.
What we’re looking for
Keep the limits of that inference. We don't know that the level stayed that high throughout the entire epoch. Nor does finding deposits from this period show that nahcolite never formed at another time. For the supported conclusion, I'd stay with the minimum we can establish: the required carbon-dioxide level existed during at least part of the Eocene.
Working the answers
- A
We haven't been given a comparison between atmospheric carbon dioxide during the Eocene and its level afterward. We also don't know the level during most of the Eocene. The evidence establishes a threshold during a particular period of formation, not the long-term history A describes.
- B
The level could have remained steady at, say, 1,500 parts per million while the deposits formed. That would satisfy the stated requirement without any large fluctuations. Knowing a threshold was met doesn't tell us how much the level varied.
- C
Salty water is part of the setting in which the formation requirement applies. Nothing says that higher carbon dioxide makes lakes salty or makes salty lakes more common. C introduces a relationship between carbon dioxide and lake salinity that the passage doesn't establish.
- DCorrect
If the carbon-dioxide level had stayed below 1,125 parts per million throughout the Eocene, nahcolite couldn't have formed in the salty lakes as described. Since it did form, the atmosphere met or exceeded that level during at least some part of the epoch. “At least” includes exactly 1,125; we don't need a higher number.
- E
The passage tells us when these large deposits formed, not that this was the only period in which significant deposits could form. If the needed conditions existed later, later deposits might have formed too. We have no evidence ruling that out.
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