In what way does hemoglobin act as a buffer against changes in blood pH?

A) Subsequent binding of oxygen is drastically reduced after the first one is bound.
B) Hemoglobin binds some of the excess protons released by carbonic acid.
C) Hemoglobin removes excess protons from the red blood cells so that they can be excreted through the kidneys.
D) Hemoglobin produces protons or hydroxide ions as needed to alter the blood pH.
E) All of the above answers apply.