Which of the following is a greenhouse gas?

• A. Oxygen
• B. Nitrogen
• C. Methane
• D. Argon

Answer: (C)

Greenhouse gases trap heat in the atmosphere by absorbing infrared radiation emitted by the Earth's surface and re-radiating it back, which warms the lower atmosphere. Methane is a greenhouse gas because its C–H bonds absorb infrared radiation strongly, so even at low concentrations it contributes significantly to the greenhouse effect. This makes methane more potent per molecule than many other gases, though it is less abundant than carbon dioxide. The other options—oxygen, nitrogen, and argon—don’t act as greenhouse gases because their molecules don’t absorb infrared radiation effectively, so they don’t trap heat in the atmosphere.