PrepTest 141, Section 4, Question 8
Fluoride enters a region's groundwater when rain dissolves(44 more words not shown)
Which one of the following(12 more words not shown)
- AFluoride-bearing minerals are(9 more words not shown)
- BRainfall does not(5 more words not shown)
- CSodium-bearing minerals dissolve(7 more words not shown)
- DSodium in groundwater(8 more words not shown)
- ESoil that contains(12 more words not shown)
Reading the argument
The sodium seems to be doing something. What else explains more fluoride when the other relevant conditions stay the same?
First follow the process we're given: rain dissolves minerals in the soil, releasing fluoride into the groundwater. Then compare the study's regions. The researchers hold rainfall and the concentration of fluoride-bearing minerals constant, along with the other relevant variables. Yet the groundwater has significantly more fluoride where it also has more sodium.
We don't need to invent a chemical explanation from outside the passage. We do need to notice what the controls accomplish. We can't explain this difference by saying one area simply got more rain or started with more fluoride-bearing minerals; those factors were held constant.
What we’re looking for
The question asks what can most reasonably be concluded. I'd expect something connecting the sodium with the increase in fluoride. The exact mechanism may be unfamiliar, but a reasonable answer should fit both the observed increase and the stated way fluoride enters the water.
Working the answers
- A
We don't have a breakdown of the groundwater's fluoride sources. The passage gives us a route through fluoride-bearing minerals, not evidence that some other source supplies more. A makes a claim about the primary source that the study doesn't establish.
- B
Holding rainfall constant doesn't show that rainfall has no effect. Rain is part of the process that releases the fluoride in the first place. The control lets researchers examine a difference while setting rainfall aside as an explanation for that difference.
- C
Comparing how fast two kinds of minerals dissolve doesn't explain why more sodium goes with more fluoride under otherwise comparable conditions. Even if sodium-bearing minerals dissolve faster, we still need the connection to the release of fluoride. C doesn't supply that connection.
- DCorrect
If sodium makes fluoride-bearing minerals dissolve faster, more fluoride can enter the groundwater under the same rainfall and mineral conditions. That fits the controlled comparison and the process described at the start. It is the most reasonable conclusion among these choices; we don't have to know the chemistry independently or treat the study as absolute proof of every detail of the mechanism.
- E
The researchers held the concentration of fluoride-bearing minerals constant. More of those minerals in the soil therefore doesn't explain the observed difference. We also aren't told that high sodium in groundwater means a high concentration of sodium-bearing minerals in the soil. E changes the comparison rather than explaining the study's result.
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