Answer:
D. Type II alveolar cells
Explanation:
Three layers together make the diffusion membrane or respiratory membrane, which make the total thickness of less than a millimeter i.e. about 0.2 micrometer. The three layers are as follows:
1. The squamous epithelium of alveoli. There are two types of alveolar cells called type I and type II. Type I makes the exchange surface of alveolar membrane and type II cells are responsible for secreting the lung surfactant that reduces the surface tension.
2. Endothelial lining of blood capillaries. These is rich network of blood capillaries near the wall of alveoli.
3. In between the squamous epithelium and the endothelium of blood capillaries fused basement membrane is present.
Monomer is only one molecule, where as polymers are multiple monomers connected
