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: Dental
In which of the following case Hamman's sign is seen? 
Select and explain. 
A. Hematoma, B. Accidental  NaOCl, C. Pneumomediastinitis, D. Oral carcinoma
C: Pneumomediastinitis
Answer: C: Pneumomediastinitis 
Explanation: None