2021 O Level Chem P1 Q30 — Air and the Atmosphere / Pollutants Solution

Answer: B. The correct row pairs carbon monoxide with the incomplete combustion of petrol, and nitrogen dioxide with oxygen and nitrogen combining inside an internal combustion engine. Both pollutant–source pairings are right.

Carbon monoxide forms when petrol burns in too little oxygen (incomplete combustion). At the high temperatures inside an engine, nitrogen and oxygen from the air react to form oxides of nitrogen such as NO₂.

The other rows pair a pollutant with the wrong source: carbon dioxide is not made by photosynthesis (photosynthesis removes CO₂); methane is not from petrol combustion; and sulfur dioxide comes from burning fossil fuels containing sulfur, not from lightning (lightning produces oxides of nitrogen).