The esophagus, a tube that carries food to the stomach, is constantly being scraped as food travels through it. Why does the esophagus not deteriorate through a person’s lifetime? 1. Cells can anabolize the food prior to entering the esophagus.
2. Cells are constantly dividing and replacing old or damaged cells.
3. Cells promote a fluid to build up along the esophagus to reduce friction.
4. Cells build a type of protein called enzymes that act as an armor.

i think the answer is b.

the nitrogen cycle provides nitrogen to the ecosystem from the atmosphere, ground and oceans. nitrogen is an important component of complex molecules such as amino acids and nucleotides, which lead to the creation of proteins and dna, the building blocks of all life. plants absorb nitrogen in the form of ammonium, nitrate ion and, on rare occasions, as amino acids. animals receive nitrogen necessary for biological processes from feeding on living or dead organic matter.