Which gas do animals breathe in that is essential for life?

• A. Carbon dioxide
• B. Hydrogen
• C. Nitrogen
• D. Oxygen

Answer: (D)

Animals breathe in oxygen, which is essential for life because it plays a vital role in the process of cellular respiration. During cellular respiration, oxygen is used by cells to convert nutrients from food into energy. This energy is crucial for various bodily functions, growth, and maintaining life.

While carbon dioxide and nitrogen are also present in the air, carbon dioxide is actually a waste product that animals exhale after their bodies use oxygen, and nitrogen, although abundant in the atmosphere, does not participate directly in the metabolic processes that provide energy to living organisms. Hydrogen is not commonly inhaled in significant amounts and is not involved in the respiration process in the same way that oxygen is. Therefore, oxygen is the gas that is essential for animals to sustain life.