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
O2 delivery to tissue is decreased by: 
Select and explain. 
A. 4,ed Haemoglobin level, B. Ted HCO3, C. Ted Pa CO2, D. All
A: 4,ed Haemoglobin level
Answer: A: 4,ed Haemoglobin level 
Explanation: A i.e. 4-ed Hemoglobin level