Fetal Hemoglobin Has a Somewhat Higher Oxygen-Binding Affinity Than Does

Fetal hemoglobin has a somewhat higher oxygen-binding affinity than does adult hemoglobin because:

A) It has only one subunit, and therefore there is no cooperativity of oxygen binding.
B) 2,3-BPG binds less tightly to fetal hemoglobin than to adult hemoglobin, and this weakens its effect on the oxygen-binding affinity.
C) The noncovalent interactions between the subunits are stronger in fetal hemoglobin than in adult hemoglobin.
D) Fetal hemoglobin is less acidic than adult hemoglobin, and therefore it is less affected by increased acidity in the erythrocyte.
E) The terminal amino groups of fetal hemoglobin are chemically modified, and therefore they cannot react with carbon dioxide.