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
[{'content': 'In a variable performance face mask \nSelect and explain. \nA. 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.', 'role': 'user'}, {'content': 'Answer: C \nExplanation: During expiratory pause, there is no tidal\xa0 flow from the patient, oxygen accumulates in the mask, thereby washing out carbon dioxide.', 'role': 'assistant'}]