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': 'A herd immunity of over - % is considered necessary to prevent epidemic spread of diphtheria - \nSelect and explain. \nA. 50%, B. 55%, C. 60%, D. 70%', 'role': 'user'}, {'content': 'Answer: D \nExplanation: A herd immunity of over 70% is considered necessary to prevent epidemic spread, but some believe that the critical level may be as high as 90%.', 'role': 'assistant'}]