Which Biological Membranes Must Atmospheric Molecules of Oxygen Cross to Become

Which biological membranes must atmospheric molecules of oxygen cross to become bound to hemoglobin for transport in mammals?

A) one membrane (the lining in the lungs)
B) two membranes (in and out of the cell lining the lung)
C) four membranes (in and out of the cell lining the lung, in and out of the endothelial cell lining an alveolar capillary)
D) five membranes (in and out of the cell lining the lung, in and out of the endothelial cell lining an alveolar capillary, and into the red blood cell)