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': 'Salk vaccine is a \nSelect and explain. \nA. Live vaccine, B. Live attenuated vaccine, C. Toxoid, D. Killed vaccine', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]