Which Part of a Hemoglobin Molecule Binds to a Molecule $\alpha$

Multiple Choice

Which part of a hemoglobin molecule binds to a molecule of O₂?

A) the hydrophobic pocket between two $\alpha$ chains
B) the copper atom wedged between an $\alpha$ and a $\beta$ globin chain
C) the iron atom in the center of a heme group
D) the surface of a $\beta$ globin chain