Which of the following characteristics makes hemoglobin's structure such a good match for its function as an oxygen carrier?

A) Each hemoglobin molecule can bind one oxygen molecule.
B) Each hemoglobin molecule can bind two oxygen molecules.
C) Each hemoglobin molecule can bind four oxygen molecules.
D) Each hemoglobin molecule can't be saturated by oxygen molecules.
E) Each hemoglobin binds irreversibly to an oxygen molecule.