Which of the following best represents the approximate percentage of oxygen by volume in Earth's atmosphere?

• A. 50%
• B. 10%
• C. 21%
• D. 78%

Answer: (C)

Verified step by step guidance

1. Understand that Earth's atmosphere is primarily composed of nitrogen, oxygen, and other gases, with oxygen making up a significant but not majority portion by volume.
2. Recall or research the typical composition of dry air by volume, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases such as argon and carbon dioxide.
3. Recognize that the percentage of oxygen by volume is about 21%, which is less than half but more than 10%, making it a distinct and well-known value.
4. Compare the given options (50%, 10%, 21%, 78%) to the known atmospheric composition and identify that 21% best matches the approximate oxygen content.
5. Conclude that the approximate percentage of oxygen by volume in Earth's atmosphere is 21%, based on standard atmospheric composition data.