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. Which one of the following can most reasonably be concluded on the basis of the researchers' findings? ['Sodium in groundwater increases the rate at which fluoride-bearing minerals dissolve.', 'Fluoride-bearing minerals are not the primary source of fluoride found in groundwater.', 'Rainfall does not affect fluoride concentrations in groundwater.', 'Sodium-bearing minerals dissolve at a faster rate than fluoride-bearing minerals.']