What Structural Feature Is Shared by Both Capillaries and Alveoli

What structural feature is shared by both capillaries and alveoli that aids in the exchange of gasses by diffusion?

A) Both have thin walls that are just nearly one cell thick.
B) Both have small plasma membranes.
C) Both are full of oxygen-depleted blood for gas exchange.
D) Both are supplied with osmotic pressure from the diaphragm.
E) Both are uniquely thick walled to allow quick diffusion of gasses.