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: Physiology
Which of the following is true about ventilation and perfusion in alveoli in an erect posture 
Select and explain. 
A. Ventilation/perfusion ratio is maximum at apex, B. Ventilation/perfusion ratio is maximum at base, C. Ventilation is maximum at apex, D. Perfusion is maximum at apex
A: Ventilation/perfusion ratio is maximum at apex
Answer: A: Ventilation/perfusion ratio is maximum at apex 
Explanation: Apical alveoli have low ventilation and perfusion per unit volume of the lung, whereas the basal alveoli have high ventilation and perfusion.
Ventilation-perfusion ratio and the PaO2 is maximum at the apex. PCO2 is maximum at the base.