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
In which of the following conditions the respiratory muscles are relaxed? 
Select and explain. 
A. Residual volume, B. Functional residual capacity, C. Expiratory reserve volume, D. Inspiratory reserve volume
A: Residual volume
Answer: A: Residual volume 
Explanation: The volume of gas remaining in the lungs after a full expiratory effo.Ref: Ganong&;s Review of Medical Physiology; 24th edition; page no: 629