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. Cohort study, C. Ecological study, D. Cross sectional study', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]