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
Hamman's sign is positive in emphysema. 
Select and explain. 
A. Mediastinal, B. Surgical, C. Panacinar, D. Interstitial
A: Mediastinal
Answer: A: Mediastinal 
Explanation: Mediastinal