This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Communicable diseases, Subject name: Social & Preventive Medicine
[{'content': 'Most common source of Diphtheria- \nSelect and explain. \nA. Case, B. Carrier, C. Both, D. None', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Carriers are common sources of infection, their ratio is estimated to be 95 carriers for 5 clinical cases. Ref:PARK&;S TEXTBOOK OF PREVENTIVE AND SOCIAL MEDICINE 23rd edition , Page 160', 'role': 'assistant'}]