Fluoride enters a region's groundwater when rain dissolves fluoride-bearing minerals in the soil. In a recent study, researchers found that when rainfall, concentrations of fluoride-bearing minerals, and other relevant variables are held constant, fluoride concentrations in groundwater are significantly higher in areas where the groundwater also contains a high concentration of sodium.
This stimulus gives us facts rather than an argument, so we'll read carefully and push the facts together as we go.
The first sentence tells us how fluoride gets into groundwater: rain dissolves fluoride-bearing minerals in the soil, and those dissolved minerals end up in the groundwater below. So three factors are immediately relevant to how much fluoride ends up in the groundwater: how much rain there is, how many fluoride-bearing minerals are in the soil, and how quickly those minerals dissolve.
Then we learn about a study. Researchers compared different areas and found that fluoride levels in the groundwater are significantly higher where sodium levels in the groundwater are also high. That's interesting on its own, but the real power of this finding comes from what the researchers controlled for. They held rainfall, concentrations of fluoride-bearing minerals, and other relevant variables constant.
That's a well-done study. If we're controlling for rain, concentration of fluoride-bearing minerals, and other factors, then those factors can't explain why one area has more fluoride in its groundwater than another. The only variable that differs between the areas is the sodium concentration in the groundwater. That's strong evidence that the different concentrations of sodium in groundwater causally contribute to the different levels of fluoride concentrations in groundwater.
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Now, how exactly does sodium lead to more fluoride? We know from the first sentence that fluoride enters groundwater through the dissolution (dissolving) of fluoride-bearing minerals. And since the amount of fluoride-bearing minerals is the same across the areas being compared (because it's controlled for), the difference can't be that some areas just started with more minerals. This suggests the sodium in the groundwater must be affecting the process by which fluoride-bearing minerals dissolve and release fluoride into the water. We don't know the exact mechanism, but sodium is playing some role in that process.
Because this is a Most Strongly Supported question, we should be prepared to use process of elimination. But the controlled nature of this study does point us in a direction: the findings are strong evidence that sodium in the groundwater has a causal effect on fluoride concentration in groundwater.
Which one of the following can most reasonably be concluded on the basis of the researchers' findings?
Fluoride-bearing minerals are not the primary source of fluoride found in groundwater.
We're told that fluoride enters groundwater when rain dissolves fluoride-bearing minerals. But the stimulus never tells us whether there are other ways fluoride can get into groundwater. Without knowing whether other sources of fluoride exist, we can't say whether fluoride-bearing minerals are or aren't the primary source.
Rainfall does not affect fluoride concentrations in groundwater.
The first sentence tells us that fluoride enters groundwater when rain dissolves fluoride-bearing minerals in the soil.
You might be drawn to (B) because the study held rainfall constant, which could make you think rainfall doesn't matter. But controlling for a variable doesn't mean the variable has no effect. It means the researchers are deliberately removing rainfall's effect from the comparison so they can isolate the effect of sodium. The first sentence already establishes that rainfall plays a role.
Sodium-bearing minerals dissolve at a faster rate than fluoride-bearing minerals.
The stimulus doesn't mention sodium-bearing minerals at all. It tells us about sodium in the groundwater, but that's different from sodium-bearing minerals in the soil. We have no idea how the sodium got into the groundwater, so we can't draw any conclusions about sodium-bearing minerals. On top of that, the stimulus says nothing about the comparative rate at which different types of minerals dissolve.
Sodium in groundwater increases the rate at which fluoride-bearing minerals dissolve.
This is strongly supported by the study's findings. We know fluoride gets into groundwater when rain dissolves fluoride-bearing minerals. The study found that high-sodium areas have higher fluoride concentrations, even when controlling for rainfall, mineral concentrations, and other relevant variables. So what could explain the difference?
It can't be that high-sodium areas simply have more fluoride-bearing minerals to start with, because the study controlled for that. It can't be that high-sodium areas get more rain, because the study controlled for that too. The remaining explanation is that sodium is affecting the dissolution process itself. And since the study is comparing areas under the same conditions, time is effectively held constant as well. That's what strongly suggests the rate of dissolution (how quickly the fluoride dissolves) is how the sodium affects fluoride concentrations in groundwater.
Soil that contains high concentrations of sodium-bearing minerals also contains high concentrations of fluoride-bearing minerals.
Like (C), this talks about sodium-bearing minerals in the soil. But the stimulus mentions only sodium in the groundwater. We don't know how the sodium got into the groundwater, so we can't conclude anything about sodium in the soil.
There's also a second problem. (E) is essentially saying that areas with lots of sodium-bearing minerals also have lots of fluoride-bearing minerals. If that were true, it might seem to explain the study's results: maybe high-sodium areas just happen to have more fluoride minerals, and that's why there's more fluoride in the water. But the study controlled for concentrations of fluoride-bearing minerals. The areas being compared had equal amounts of fluoride minerals, and the high-sodium areas still had more fluoride in the groundwater. So (E) isn't a reasonable explanation for the study's results.
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