UPSC Prelims 2025 · GS Paper I
Question 40: Environment and Ecology
With reference to the planet Earth, consider the following statements :
- IRain forests produce more oxygen than that produced by oceans.
- IIMarine phytoplankton and photosynthetic bacteria produce about 50% of world’s oxygen.
- IIIWell-oxygenated surface water contains several folds higher oxygen than that in atmospheric air.
Which of the statements given above is/are correct?
AnswerThe answer is (b).
Marine phytoplankton and photosynthetic bacteria such as cyanobacteria produce roughly half of the oxygen in the atmosphere, so Statement II is correct. This also shows that rain forests do not produce more oxygen than the oceans, which makes Statement I wrong; much of the oxygen forests produce is used up again by their own respiration and decay. Statement III is wrong because oxygen dissolves only sparingly in water, so even well-oxygenated surface water holds many times less oxygen than the same volume of air.
- I. The oceans produce about as much oxygen as all land plants, and more than rain forests.
- II. Marine phytoplankton and photosynthetic bacteria produce about half of the world's oxygen.
- III. Water holds far less oxygen per volume than air.
Why each option is right or wrong
- (a) Wrong: statement I is incorrect, since the oceans produce about half of the oxygen.
- (b) Right: only the claim that phytoplankton and bacteria give about half of the oxygen holds.
- (c) Wrong: statements I and III are both incorrect.
- (d) Wrong: statement II is correct.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2025, General Studies Paper I, Series A. The paper on upsc.gov.in.