Which of the following features of the alveolar sacs allows for the ease of diffusion of gasses?
a. They are made of a single layer of simple squamous epithelium.
b. Type II alveolar cells secrete surfactant.
c. Elastin fibers allow the alveoli to stretch thin enough for diffusion to occur.
d. They are made of a single layer of simple squamous epithelium and elastin fibers allow the alveoli to stretch thin enough for diffusion to occur.

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