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
In which of the following a reduction in arterial oxygen tension occurs? 
Select and explain. 
A. Anaemia, B. CO poisoning, C. Moderate exercise, D. Hypoventilation
D: Hypoventilation
Answer: D: Hypoventilation 
Explanation: Hypoventilation leads to hypoxic hypoxia where both arterial oxygen content and PO2 is low.