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 true about cohort studies except - \nSelect and explain. \nA. Prospective, B. Useful for rare diseases, C. Necessary for incidence, D. Costly', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]