Question ID:PT73 S4 Q8 Passage: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. Stem:Which one of the following can most reasonably be concluded on the basis of the researchers' findings? Correct Answer Choice:DChoice A:Fluoride-bearing minerals are not the primary source of fluoride found in groundwater. Choice B:Rainfall does not affect fluoride concentrations in groundwater. Choice C:Sodium-bearing minerals dissolve at a faster rate than fluoride-bearing minerals. Choice D:Sodium in groundwater increases the rate at which fluoride-bearing minerals dissolve. Choice E:Soil that contains high concentrations of sodium-bearing minerals also contains high concentrations of fluoride-bearing minerals.