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': 'First case in an epidemic is called - \nSelect and explain. \nA. Index case, B. Primary case, C. First case, D. None of the above', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]