Which Statement About the Binding of Oxygen by Hemoglobin Is

Which statement about the binding of oxygen by hemoglobin is true?

A) The percentage of completely O₂-saturated hemoglobin decreases as PO₂ increases.
B) The O₂-binding/dissociation curve is U-shaped.
C) Over the full length of the O₂-binding/dissociation curve, O₂ dissociation-association is rapid.
D) A very low PO₂ is required for all four hemoglobin subunits to bind to O₂.
E) Once one subunit of hemoglobin binds to O₂, the affinity of the remaining subunits for their O₂ increases until three O₂ are bound.