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
All of the following factors influence hemoglobin dissociation curve, except 
Select and explain. 
A. Chloride ion concentration, B. CO2 tension, C. Temperature, D. 2–3 DPG levels
A: Chloride ion concentration
Answer: A: Chloride ion concentration 
Explanation: Hb-O2 dissociation curve shift to left indicates more affinity of Hb to O2 and thus less delivery of O2 to tissues.
Shift to left seen in
-        Increased pH
-        Decreased 2,3 DPG, temperature, PCO2
-        HbF (Fetal)
Shift to right seen in
-        Decreased pH
-        Increased 2,3DPG, temperature, PCO2
-        HbS (Sickle cell)