Passage Summary: Quitting smoking is hard because it makes people stressed and causes them to gain weight. A study found that people who exercised while trying to quit had a much higher success rate than those who didn't, suggesting that exercise might be the secret to quitting.

Conclusion: The key to successfully quitting smoking may be replacing the habit with a healthy activity such as aerobic exercise.

Reasoning: In a study, 40 percent of participants who added fifteen weeks of aerobic exercise to a smoking-cessation program successfully quit for a month, while none of the participants in the cessation program alone were successful.

Analysis: The argument relies on a correlation from a study to suggest that exercise is the causal 'key' to quitting smoking. To strengthen this, we want to see evidence that exercise directly addresses the obstacles mentioned—stress and weight gain—or that the study's two groups were identical in every other way. Quitting smoking is notoriously difficult, mostly because humans generally prefer immediate gratification over long-term lung health, so any factor that tips the scales is significant. Since this is a 'Strengthen EXCEPT' question, look for the one answer choice that fails to support the link between exercise and quitting success, perhaps by being irrelevant or by introducing a flaw in the study's design.