Passage Summary: A dentist argues that putting fluoride in water doesn't really work because kids in Europe (where they don't do it) have better teeth than kids in the US (where they do).

Conclusion: Adding fluoride to water does not significantly help in preventing tooth decay.

Reasoning: Data from five studies show that children in non-fluoridated European regions actually have fewer dental issues than children in the fluoridated United States.

Analysis: The dentist is making a causal claim based on a broad correlation between two different geographic populations. To strengthen this, we need to eliminate 'confounding variables'—other reasons why Europeans might have better teeth, such as lower sugar intake or better overall dental hygiene habits. If we find out that these other factors are actually the same in both places, it makes the lack of fluoridation look like a more plausible explanation. Look for an answer that makes the two groups as similar as possible in every way except for the water they drink.