The air we breathe consists mostly of which gas?

• A. Argon (Ar)
• B. Oxygen (O_2)
• C. Carbon dioxide (CO_2)
• D. Nitrogen (N_2)

Answer: (D)

Verified step by step guidance

1. Identify the major components of air by volume. Air is a mixture of gases, primarily nitrogen, oxygen, argon, carbon dioxide, and trace gases.
2. Recall that nitrogen (N_2) makes up about 78% of the Earth's atmosphere by volume, making it the most abundant gas in the air we breathe.
3. Recognize that oxygen (O_2) is the second most abundant gas, comprising about 21% of air, which is essential for respiration but not the majority component.
4. Understand that argon (Ar) and carbon dioxide (CO_2) are present in much smaller amounts, roughly 1% and 0.04% respectively, and thus are not the main gases in air.
5. Conclude that since nitrogen (N_2) is the largest component by volume, the air we breathe consists mostly of nitrogen gas.