Question

the purpose of the respiratory system is to transfer oxygen from the air to the blood and to remove carbon dioxide from the blood. this process is called respiration, and breathing is part of it. our cells need oxygen to release the energy from food. when they do this, they create carbon dioxide as a waste product. this waste is eliminated by the respiratory system.
a byproduct of respiration is
choose one option
a blood.
b oxygen.
c air.
d carbon dioxide.

Explanation:

The text states that during respiration, cells use oxygen to release energy from food and create carbon dioxide as a waste (byproduct). Blood is involved in transporting substances, oxygen is used in the process, and air is the source of oxygen, so they are not byproducts. Only carbon dioxide is the byproduct.

Answer:

D. carbon dioxide.