To strengthen the link between vehicle emissions and asthma prevalence, the most persuasive evidence would be

• A. A theoretical model with no data.
• B. Longitudinal studies controlling for confounders showing a correlation.
• C. An opinion poll about air quality.
• D. A description of air fresheners.

Answer: (B)

Longitudinal studies that follow people over time and carefully control for other factors provide the strongest basis for linking vehicle emissions to asthma prevalence, because they show the sequence of exposure followed by outcome and reduce the impact of confounding variables. By adjusting for things like other air pollutants, socioeconomic status, smoking, and housing conditions, the observed association becomes more credible and less likely to be due to external factors. In contrast, a theoretical model without data offers no real-world evidence, an opinion poll only captures perceptions about air quality, and a description of air fresheners has no direct relevance to vehicle emissions and asthma.