Correct answer: (d) 1, 2, 3 and 4
Explanation
- A
1 and 2 only
(a) 1 and 2 only. Crop-residue burning emits carbon monoxide and methane, but ozone (as a secondary pollutant from precursors) and sulphur dioxide are also in the mix, so 1-and-2 is not the key.
- B
2, 3 and 4 only
(b) 2, 3 and 4 only. Drops carbon monoxide, a primary incomplete-combustion gas from stubble fires, so this is not the key.
- C
1 and 4 only
(c) 1 and 4 only. Incomplete relative to the full emission set, so not the key.
- D
1, 2, 3 and 4
(d) 1, 2, 3 and 4. Open burning of crop/biomass residue releases CO, methane, SO2 and precursors that form ozone, along with PM2.5. All four listed gases are therefore associated with that burning, and this is the official key.
Summary. Official key is (d) all four. Stubble and biomass fires are incomplete combustion: carbon monoxide, methane, sulphur dioxide and ozone-related photochemistry plus particulates. North Indian winter smog questions often sit next to this fact. Honour the stored letter (d).
Same topic · past papers
UPSC has asked this before
These previous-year questions sit on the same topic. Open one to practise the earlier ask.
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