The primary job of the respiratory system is respiration, or breathing. In humans and other animals, breathing is how oxygen moves into the lungs. The human body can store many nutrients, but it can’t store oxygen. Oxygen is essential for all cells to function properly. That’s why humans have to breathe continually to stay alive. Respiration also moves carbon dioxide, a waste product, out of the body. Which of these best describes the relationship between respiration and oxygen?

A
Oxygen is the process by which respiration happens.

B
Respiration and oxygen are both types of gases.

C
Respiration is how the body takes in oxygen.

D
Oxygen comes before respiration.