This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
[{'content': 'To test the association between risk factor and disease, which of the following is the weakest study design - \nSelect and explain. \nA. Case-control study, B. Ecological study, C. Cohort study, D. Cross-sectional study', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]