A
0°C
B
–18°C
C
5°C
D
–20°C
Correct answer: (b) –18°C
Explanation
- A
0°C
(a) 0°C. Zero is the ice point of water, not the no-greenhouse planetary mean.
- B
–18°C
(b) –18°C. Without the natural greenhouse effect Earth’s average surface would be about –18°C (255 K). The actual mean is about +15°C. That textbook figure is the official key.
- C
5°C
(c) 5°C. Too warm for a no-greenhouse Earth and not the standard value.
- D
–20°C
(d) –20°C. Close in magnitude but not the conventional –18°C figure used in Indian exam keys.
Summary. Official key is (b). Without greenhouse gases the global mean surface temperature would be about –18°C, not 0°C, 5°C or –20°C.
Same topic · past papers
UPPSC has asked this before
These previous-year questions sit on the same topic. Open one to practise the earlier ask.