This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Mass vaccination is ineffective in - \nSelect and explain. \nA. Measles, B. Polio, C. Tetanus, D. None', 'role': 'user'}, {'content': 'Answer: A \nExplanation: .', 'role': 'assistant'}]