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
Hemoglobin unlike myoglobin showsa) Sigmoid curve of oxygen dissociationb) Positive cooperativityc) Hills coefficient of oned) None of above 
Select and explain. 
A. ac, B. ab, C. ad, D. bc
B: ab
Answer: B: ab 
Explanation: Hb-O2 dissociation curve- Sigmoid shape
Co-operativity- Hb is a tetramer. If one molecule of O2 binds to the tetramer more easily if other O2 is already bound. 
 
Hills coefficient of Hb- 2.8