In a variable performance face mask: (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.
Answer is C. During expiratory pause, there is no tidal  flow from the patient, oxygen accumulates in the mask, thereby washing out carbon dioxide.