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: Medicine
Shifting dullness is the characteristic sign of: 
Select and explain. 
A. Hydropneumothorax, B. Pneumothorax, C. Consolidation, D. Collapse
A: Hydropneumothorax
Answer: A: Hydropneumothorax 
Explanation: None