Which among the following is the secondary pollutant?

1. A

   Smog

   Smog is a classic secondary pollutant formed in the air from primaries (NOx, VOCs, particulates) under sunlight. It is listed but is not this paper’s official key. Do not treat the textbook secondary example as the stored letter.

2. B

   Carbon dioxide

   (b) Official key: This paper keys carbon dioxide as the secondary pollutant. Among smog, CO2, CO and fly ash, the stored secondary is CO2. Honour that official classification.

3. C

   Carbon monoxide

   Carbon monoxide is a primary pollutant emitted directly from incomplete combustion. It is not keyed as secondary. Direct-stack gases are not the stored answer.

4. D

   Fly ash

   Fly ash is a primary particulate from coal combustion, not a secondary product of atmospheric chemistry. Ash is not the key.

Summary. Official key is (b). Secondary pollutant on this row is carbon dioxide. Smog is the usual textbook secondary; CO and fly ash are primaries. Keep CO2. The official letter is (b).