Answer: D. Red blood cells contain a protein that binds with oxygen.
Explanation: Each RBC contains millions of molecules of hemoglobin, a complex protein designed to bind and release oxygen. When hemoglobin binds with oxygen, it’s called oxyhemoglobin. RBCs take oxygen to body tissues, where the oxygen is released. As the tissues use oxygen, they produce the waste product carbon dioxide, which is picked up by more RBCs and brought back to the lungs for release out of the body via exhalation.