Use the passage below to answer the question below the passage. 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.

Question: A byproduct (effect) of respiration is.

A. blood.

B. oxygen.

C. carbon dioxide.

D. air.