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
[{'content': 'The sigmoid nature of Hb-O2 dissociation curve is because of \nSelect and explain. \nA. Binding of one O2 molecule increase the affinity for the next O2 molecule, B. Alpha chain has more affinity for O2 than beta chain, C. Beta chain has more affinity for O2 than alpha chain, D. Hemoglobin is acidic in nature', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]