When oxygen binds to a heme-containing protein, the two open coordination bonds of fe2+ are occupied by: ?

I believe that the two open coordination bonds of Fe2+ are occupied by one oxygen molecule and one amino acid atom. 
Heme protein is a protein that contains a heme prosthetic group. The heme group confers functionality, which can include oxygen carrying, oxygen reduction, electron transfer, and other processes. Hemoglobin contains four heme units in total, allowing four oxygen atoms in total to bind to the protein.

The answer is observations and interpreting.

Ithink your answer will be c let me know if it’s right