Which gas makes up approximately 21% of Earth's atmosphere?

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

Answer: (C)

Verified step by step guidance

1. Understand that Earth's atmosphere is composed of several gases, each with a specific percentage by volume.
2. Recall the major components of the atmosphere: Nitrogen (~78%), Oxygen (~21%), Argon (~0.93%), and Carbon dioxide (~0.04%).
3. Identify that the question asks for the gas that makes up approximately 21% of the atmosphere.
4. Match the given options with their typical atmospheric percentages: Nitrogen (~78%), Oxygen (~21%), Carbon dioxide (trace amounts), Argon (~0.93%).
5. Conclude that Oxygen is the gas that constitutes about 21% of Earth's atmosphere.