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
Increased airway resistance is/are caused bya)  Forced expirationb) Denser airc)  Low lung volumed) High lung volue 
Select and explain. 
A. ab, B. bc, C. acd, D. abc
D: abc
Answer: D: abc 
Explanation: There is dynamic compression of the small airways during forceful expiration, resulting in increased airway resistance. When the air is denser, the airway resistance is more. Less volume of the lung also increases the airway resistance.