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: Physiology
The intrapleural pressure is negative both during inspiration and expiration because: 
Select and explain. 
A. Intrapulmonary pressure is always negative, B. Thoracic cage and lungs are elastic structure, C. Transpulmonary pressure determines the negativity, D. All
B: Thoracic cage and lungs are elastic structure
Answer: B: Thoracic cage and lungs are elastic structure 
Explanation: B i.e. Thoracic cage and lungs are elastic structures