Some features needed in an efficient gas exchange system are a large surface area and a short diffusion distance. Which statement correctly explains how alveoli are adapted to satisfy these requirements?
- Alarge surface area: collagen and elastin fibres that allow the alveoli to stretch; short diffusion distance: an extracellular layer round the alveolus wall contains blood capillaries
- Blarge surface area: gases dissolve in a layer of liquid to speed up diffusion; short diffusion distance: walls of alveoli are squamous epithelium
- Clarge surface area: many folded interconnected alveoli; short diffusion distance: capillaries are next to alveolus wall
- Dlarge surface area: sac shape of alveoli formed by squamous cells; short diffusion distance: red blood cells are very close to capillary walls