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': 'Tuberculin unit is: \nSelect and explain. \nA. 0.0001 mg, B. 1 unit of PPD 3, C. 0.1 mg BCG, D. None of the above', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 1 unit of PPD 3', 'role': 'assistant'}]