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': 'Study of Natural history is with \nSelect and explain. \nA. Ecological study, B. Longitudinal study, C. Cross - sectional study, D. All of the above', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]