Respond in one or two sentences explaining the key idea:
 
During a respiration experiment, a student observes that the volume of carbon dioxide released is equal to the volume of oxygen consumed by the tissue.

Based on this observation, identify the respiratory substrate being used and justify your answer using the concept of Respiratory Quotient.
 
Answer variants:
respiration
oxygen
carbon dioxide
transpiration
 
Respiratory Quotient is the ratio of the volume of
produced to the volume of
consumed during
.