Which of the following statements is correct?
A. Hemoglobin binds O2 more tightly at pH 7.2 than at pH 7.4.
B. Fetal hemoglobin under physiological conditions binds O2 more tightly than maternal hemoglobin.
C. The T state of hemoglobin binds oxygen more tightly than the R state.
D. The heterotropic effector 2,3-bisphosphoglycerate increases the binding affinity of hemoglobin for O2.
E. None of the above statements is correct.