UPSC Prelims 2018 · GS Paper I
Question 57: Geography
Consider the following statements :
- 1.The Earth’s magnetic field has reversed every few hundred thousand years.
- 2.When the Earth was created more than 4000 million years ago, there was 54% oxygen and no carbon dioxide.
- 3.When living organisms originated, they modified the early atmosphere of the Earth.
Which of the statements given above is/are correct ?
AnswerUPSC’s answer is (c).
Palaeomagnetic records show that the Earth's magnetic field has flipped many times, on average every few hundred thousand years. The early atmosphere had almost no free oxygen and held gases such as carbon dioxide, nitrogen, water vapour and methane, so 54 per cent oxygen with no carbon dioxide is wrong. The arrival of photosynthetic life changed this atmosphere over time and raised oxygen levels.
- 1. Geomagnetic reversals occur every few hundred thousand years on average.
- 2. The early atmosphere had almost no free oxygen and plenty of carbon dioxide.
- 3. Living organisms, through photosynthesis, changed the atmosphere.
How you could have got this
- If you were sure statement 2 is not correct, (b) and (d) go: each of them includes 2.
What you needed to know: The early atmosphere had almost no free oxygen and plenty of carbon dioxide.
- Then, if you were sure statement 3 is correct, (a) goes: it leaves 3 out.
What you needed to know: Living organisms, through photosynthesis, changed the atmosphere.
Only (c) is left. You didn’t need statement 1.
Which options go is worked out from UPSC’s key and the options, not by a model.
Why each option is right or wrong
- (a) Incomplete: statement 3 is also correct.
- (b) Wrong: statement 2 is false.
- (c) Right: statements 1 and 3.
- (d) Wrong: statement 2 is false.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2018, General Studies Paper I, Series A. The paper on upsc.gov.in.