The Oxygen Dissociation Curve of Hemoglobin Is Sigmoidal, and This

The oxygen dissociation curve of hemoglobin is sigmoidal, and this raises the efficiency of oxygen delivery considerably.The reason for hemoglobin's sigmoidal oxygen dissociation curve is:

A) The steric control of oxygen access to the heme iron by the distal histidine.
B) The difference in oxygen affinity between the heme groups of the α chains and those of the β chains.
C) A conformational change in the protein that raises the oxygen-binding affinities of the other heme groups when one of them becomes oxygenated.
D) The effect of 2,3-BG, which binds only to oxygenated but not deoxygenated hemoglobin.
E) The hydrophobicity of the heme-binding pocket, which forces oxygen to diffuse through a hydrophobic medium in order to reach the heme iron.