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UPSC CSE CSAT 2016 · Question 67 · Reading comprehension

Which among the following is the most logical corollary to the above passage?

Passage—4

An increase in human-made carbon dioxide in the atmosphere could initiate a chain reaction between plants and microorganisms that would unsettle one of the largest carbon reservoirs on the planet – soil. In a study, it was found that the soil, which contains twice the amount of carbon present in all plants and Earth's atmosphere combined, could become increasingly volatile as people add more carbon dioxide to the atmosphere. This is largely because of increased plant growth. Although a greenhouse gas and a pollutant, carbon dioxide also supports plant growth. As trees and other vegetation flourish in a carbon dioxide-rich future, their roots could stimulate microbial activity in soil that may in turn accelerate the decomposition of soil carbon and its release into the atmosphere as carbon dioxide.

CSAT 2016 · Q67

Reading comprehension Medium

Which among the following is the most logical corollary to the above passage?

Answer & explanation

Answer: (d) Increasing green cover could trigger the release of carbon trapped in soil

More carbon dioxide boosts plant growth, and flourishing roots stimulate soil microbes that speed up the decomposition of soil carbon and its release as carbon dioxide. So an increase in green cover could trigger the release of carbon locked in soil.

  • ✗ (a) The passage says CO₂ 'supports plant growth'; it says nothing about it being essential for microorganisms.
  • ✗ (b) 'Solely' is too strong; human-made CO₂ is one addition, not the only source.
  • ✗ (c) Reverses the chain: plant growth stimulates the microbes, not the other way round.
  • ✓ (d) As vegetation flourishes, 'their roots could stimulate microbial activity in soil that may in turn accelerate the decomposition of soil carbon'.

Remember · Follow the chain the passage builds (CO₂ → plant growth → microbes → soil carbon released); the corollary is its end point.

Question and answer: UPSC's official GS Paper II (2016, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·

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