A patient is receiving heliox therapy with a mixture of 70% / 30% by nonrebreathing mask to reduce airway resistance. The therapist also observes the reservoir bag is not collapsing at all with each inspiration. After ensuring the face mask is sealed around the face properly, the therapist should
a. obtain a partial rebreathing mask
b. discard the valve between the mask and the reservoir
c. obtain an arterial blood gas
d. reduce the total flow of gas mixture to the patient