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: Anaesthesia
In a variable performance face mask 
Select and explain. 
A. High patient inspiratory flow rate reduces oxygen concentration, B. Oxygen concentration decreases during an expiratory pause, C. Carbon dioxide concentration decreases during an expiratory pause, D. Side holes allow venting of expired gases and entrainment of air.
C: Carbon dioxide concentration decreases during an expiratory pause
Answer: C: Carbon dioxide concentration decreases during an expiratory pause 
Explanation: During expiratory pause, there is no tidal  flow from the patient, oxygen accumulates in the mask, thereby washing out carbon dioxide.