The following are greenhouse gases except

Step 1: Analyze the question You are asked to pick the gas that is NOT a greenhouse gas. The word "except" means three options are greenhouse gases and one is not.

Step 2: Understand the concept Greenhouse gases trap heat in the Earth's atmosphere. They absorb infrared (heat) radiation that the Earth gives off and re-emit it, which warms the planet. This is the greenhouse effect. Too much of these gases causes global warming and climate change.

The main greenhouse gases are:

• Carbon dioxide (CO₂): from burning fuels, deforestation and respiration
• Methane (CH₄): from swamps, rice paddies, livestock and refuse dumps
• Chlorofluorocarbons (CFCs): from refrigerators, air conditioners and aerosol sprays
• Nitrous oxide (N₂O) and water vapour

Step 3: Check each option

• A. Carbon dioxide: a greenhouse gas.
• B. Chlorofluorocarbon: a greenhouse gas. It is also well known for destroying the ozone layer.
• C. Methane: a greenhouse gas.
• D. Oxygen (O₂): NOT a greenhouse gas. Simple diatomic molecules like O₂ and N₂ do not absorb infrared radiation significantly, so they do not trap heat.

Step 4: Answer The correct option is D, oxygen.

Tip: Greenhouse gases are usually molecules with three or more atoms, or complex molecules (CO₂, CH₄, CFCs, N₂O, H₂O). The two most abundant gases in air, nitrogen and oxygen, are not greenhouse gases. Also remember not to confuse the greenhouse effect (CO₂, CH₄) with ozone depletion (CFCs), because CFCs are involved in both.