CO Binds More Strongly to the Heme Iron Than O₂

Multiple Choice

CO binds more strongly to the heme iron than O₂ does due to which reason?

A) CO binds to the heme iron perpendicular to the porphyrin ring, allowing maximum electron orbital overlap between CO and Fe²⁺.
B) CO weighs less than O₂, therefore binding to the heme iron more easily.
C) CO binds to the heme iron covalently, while O₂ binds via ionic interactions.
D) All of the answers are correct.
E) None of the answers is correct.