Transform the observation 'People who eat late-night snacks have higher BMI' into a refined, testable hypothesis suitable for a randomized trial.

• A. Adults aged 25–50 randomized to avoid consuming any calories after 8 p.m. for 6 months will experience a greater mean reduction in BMI (change ≥0.5 kg/m^2) compared with adults randomized to continue their usual late-night snacking habits, controlling for total daily calorie intake and physical activity
• B. People who snack at night tend to gain weight sometimes, so a public health campaign telling everyone never to eat after 8 p.m. is guaranteed to reduce BMI across populations
• C. If you remove snacks completely from a diet, BMI will drop instantly regardless of other factors and no control group is needed
• D. Late-night snacking is bad and people should stop to lower BMI because observational data show a relationship

Answer: (A)