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': 'True of case control studies - \nSelect and explain. \nA. It proceeds from cause to effect, B. Odds ratio can be calculated, C. Incidence can be calculated, D. Needs many patients', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]