An argument proposes: 'To improve public health, the city should invest heavily in building more bike lanes. A recent study in 'Health Quarterly' found that cities with more bike lanes have lower rates of obesity and heart disease.' Which of the following, if true, would *most* strengthen the argument's causal link?
✓ Correct Answer: Option A
Controlling for confounding variables in the study design strengthens the causal inference. If the study accounted for other factors, it makes the correlation between bike lanes and health outcomes more likely to be a direct cause.