The Fe²⁺ Atom in Hemoglobin Can Be Bonded to 6

The Fe²⁺ atom in hemoglobin can be bonded to 6 coordinate ligands. What are the six coordinate ligands of ferrous iron bonded to?

A) 4 to the porphyrin ring, 1 to the proximal histidine, 1 to the distal histidine
B) 4 to the porphyrin ring, 1 to the proximal histidine, 1 to oxygen
C) 4 to the porphyrin ring, 1 to the distal histidine, 1 to oxygen
D) 1 to oxygen, 5 to histidines in the hemoglobin polypeptide
E) the ferrous iron only binds to CO₂