Mucous secretions contain , a protein that increases viscosity of mucus to more effectively trap inhaled dust, dirt particles, microorganisms, and pollen.

Mucin

Explanation:

Mucus contains a protein called mucin. It is a high molecular weight, glycosylated protein. These are produced by epithelial cells of the individuals. Mucus contains proteins, carbohydrates, salts. But the key component of mucus is mucin protein.

Mucin can produce gel-like secretion which helps in trapping the foreign pathogens, bacteria, dust.  Mucin acts as a chemical barrier to the skin. The overproduction of mucin indicates the unhealthy condition of humans like bronchitis, cancer in the liver and pancreas.

Mucin is rich in cysteine amino acids. It is present in high mucus-secreting areas like nose, mouth, stomach, and gastrointestinal areas. They prevent bacterial and fungal infections by reducing biofilm formation.

for a different answer set the answer would be because the oxygen in the water had depleted (apex)

they use sunlight, water, and carbon dioxide to make sugars - c.