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
The oxygen hemoglobin dissociation curve is sigmoid because: 
Select and explain. 
A. Binding of one oxygen molecule increases the affinity of binding other 0, molecules, B. Binding of one oxygen molecule decrease the affinity of binding other 02 molecules, C. Oxygen affinity of Hemoglobin decreases when the pH of blood falls, D. Binding of oxygen to hemoglobin reduces the affinity of Hb for CO2
A: Binding of one oxygen molecule increases the affinity of binding other 0, molecules
Answer: A: Binding of one oxygen molecule increases the affinity of binding other 0, molecules 
Explanation: A i.e. Binding of one oxygen molecule increases the affinity of binding other 0, molecule