This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Physiology
Closing capacity of lung is related to 
Select and explain. 
A. Small sized bronchioles without cailage in nondependent poion of lung, B. Small sized bronchioles without cailage in dependant poion of lung, C. Medium sized bronchioles without cailage in nondependent poion of lung, D. Medium sized bronchioles without cailage in dependent poion of lung
B: Small sized bronchioles without cailage in dependant poion of lung
Answer: B: Small sized bronchioles without cailage in dependant poion of lung 
Explanation: The closing capacity (CC) is the volume in the lungs at which its smallest airways, the respiratory bronchioles, collapse. It is defined mathematically as the sum of the closing volume and the residual volume. The alveoli lack suppoing cailage and so depend on other factors to keep them open Ref: guyton and hall textbook of medical physiology 12 edition page number:324,325,326