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: Radiology
[{'content': "Hamman's sign is positive in emphysema. \nSelect and explain. \nA. Mediastinal, B. Surgical, C. Panacinar, D. Interstitial", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Mediastinal', 'role': 'assistant'}]