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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.
8.
Which one of the following can most reasonably be concluded on the basis of the researchers' findings?
a
Fluoride-bearing minerals are not the primary source of fluoride found in groundwater.
b
Rainfall does not affect fluoride concentrations in groundwater.
c
Sodium-bearing minerals dissolve at a faster rate than fluoride-bearing minerals.
d
Sodium in groundwater increases the rate at which fluoride-bearing minerals dissolve.
e
Soil that contains high concentrations of sodium-bearing minerals also contains high concentrations of fluoride-bearing minerals.