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
High air flow oxygen enriched devices 
Select and explain. 
A. Are variable performance devices, B. Deliver constant concentrations of oxygen, C. Require oxygen entrainment, D. Require closed face masks.
B: Deliver constant concentrations of oxygen
Answer: B: Deliver constant concentrations of oxygen 
Explanation: High air flow oxygen enriched devices are fixed performance devices and deliver constant concentration of oxygen. They require face masks with holes to enable high volumes of air to entrained.