Passage Summary: Scientists found that if you have two areas with the same amount of rain and the same minerals, the one with more salt in the water will also have more fluoride.

Reasoning: While rain and mineral presence are the primary drivers of fluoride in groundwater, data shows that higher sodium concentrations correlate with higher fluoride levels even when those primary factors are held constant.

Analysis: Since this is a 'Most Strongly Supported' question, we must treat the researchers' findings as absolute truth and see what they imply. The study isolated variables to show that sodium has a specific relationship with fluoride levels that isn't explained by rain or mineral density alone. We should look for an inference suggesting that sodium either facilitates the dissolution of fluoride or is otherwise chemically linked to its presence in the water.