Answer: Carbon dioxide (CO 2 ).
- A Methane (CH<sub>4</sub>)
- B Carbon dioxide (CO<sub>2</sub>)
- C Nitrous oxide (N<sub>2</sub>O)
- D Chlorofluorocarbons (CFCs)
Correct answer: B. Carbon dioxide (CO<sub>2</sub>)
Explanation: Although methane and CFCs are far more potent per molecule, CO<sub>2</sub> is present at far greater concentrations (over 420 ppm currently) and therefore contributes the largest share of total enhanced greenhouse forcing. CH<sub>4</sub> and N<sub>2</sub>O are also significant but at much lower concentrations.
Classical smog is a reducing mixture of SO₂ and fog, while photochemical smog is an oxidising mixture generated by UV-driven reactions of NOx and hydrocarbons.
Concept context
Understand how human activities alter the atmosphere, water, and soil. Covers air pollutants, smog types, ozone depletion, acid rain, greenhouse effect, water and soil pollution, and the principles of green chemistry.