Which of the following best describes the current composition of Earth's atmosphere by volume?

• A. Approximately 78% N_2, 21% O_2, 1% Ar, and trace amounts of CO_2 and other gases
• B. Approximately 70% CO_2, 20% N_2, 10% O_2
• C. Approximately 50% O_2, 30% N_2, 15% CO_2, and 5% Ar
• D. Approximately 90% O_2, 5% N_2, 5% Ar

Answer: (A)

Verified step by step guidance

1. Understand that Earth's atmosphere is primarily composed of gases by volume, with nitrogen (N_2) and oxygen (O_2) being the major components.
2. Recall that nitrogen makes up the largest portion of the atmosphere, approximately 78% by volume.
3. Recognize that oxygen is the second most abundant gas, making up about 21% by volume.
4. Note that argon (Ar) is a noble gas present in smaller amounts, roughly 1% by volume.
5. Acknowledge that carbon dioxide (CO_2) and other trace gases exist in very small amounts, typically less than 1%, completing the typical composition of Earth's atmosphere.