Explain how the basic structure of the human respiratory system provides such a large area for gas exchange

Lung alveoli are the ends of the respiratory tree which form clusters, called alveolar sacs. The air sacs have a very large total surface area and are surrounded by the large capillary network. The large surface area of the membrane is due to the folding of the membrane into about 300 million alveoli, with diameters of approximately 70-300 µm each. There is an exchange of gases between the air sacs and capillary blood vessels. O2 diffuses from the air sac into the blood and CO2 diffuses from the blood into the air sac. The air sac walls are very thin so that gases can quickly diffuse through them and moist with mucus so that gases can dissolve before diffusing. 

Your answer here is d