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': 'All are analytic studies except - \nSelect and explain. \nA. Field trials, B. Case-control, C. Cohort, D. Ecological', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]