To become bound to hemoglobin for transport in a mammal, atmospheric molecules of oxygen must cross

A) zero membranes-oxygen binds directly to hemoglobin, a protein dissolved in the plasma of the blood.
B) one membrane-that of the lining in the lungs-and then bind directly to hemoglobin, a protein dissolved in the plasma of the blood.
C) two membranes-in and out of the cell lining the lung-and then bind directly to hemoglobin, a protein dissolved in the plasma of the blood.
D) four membranes-in and out of the cell lining the lung, in and out of the endothelial cell lining the pulmonary capillary-and then bind directly to hemoglobin, a protein dissolved in the plasma of the blood.
E) five membranes-in and out of the cell lining the lung, in and out of the endothelial cell lining the pulmonary capillary, and into the red blood cell-to bind with hemoglobin.