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
B: Functional residual capacity
Answer: B: Functional residual capacity 
Explanation: (Functional residual capacity) (652- Ganong 22th) (621- K.S. 4th) (444- A.K.Jain 3rd) (593- Ganong 23rd)The lung volume at which airway pressure is 'zero' mm Hg is called relaxation volume, whch is equal to FRC.Therefore, relaxation volume is the point where the recoil of the chest and the recoil of the lungs balanceResidual volume - The amount of air remaining in the lung even after forced expiration(i) It helps to aerate the blood in between breathing and during expiration(ii) It maintains the contour of the lungs