This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System, Subject name: Physiology
Respiratory quotient of the brain is? 
Select and explain. 
A. 0.6-0.7, B. 0.7-0.8, C. 0.8-0.9, D. 0.9-1.0
D: 0.9-1.0
Answer: D: 0.9-1.0 
Explanation: Ans. D 0.9-1.0* Respiratory quotient (RQ) is the ratio in the steady state of the volume of carbon dioxide produced to the volume of oxygen consumed per unit time.* RQ of the brain is 0.97-0.99; this is because the principal fuel for the brain is carbohydrates.