Single correct option
Read the passage with a sentence removed, marked [BLANK].
"Permafrost — ground that has remained frozen for at least two consecutive years — underlies roughly a quarter of the land in the Northern Hemisphere. [BLANK] As permafrost thaws, this organic matter becomes available to microbes, which decompose it and release both carbon dioxide and methane, a considerably more potent greenhouse gas over shorter timescales. This creates a feedback loop that concerns climate scientists: warming thaws permafrost, thawing permafrost releases greenhouse gases, and those gases contribute to further warming."
Which of the following sentences most logically fills the [BLANK]?
"Permafrost — ground that has remained frozen for at least two consecutive years — underlies roughly a quarter of the land in the Northern Hemisphere. [BLANK] As permafrost thaws, this organic matter becomes available to microbes, which decompose it and release both carbon dioxide and methane, a considerably more potent greenhouse gas over shorter timescales. This creates a feedback loop that concerns climate scientists: warming thaws permafrost, thawing permafrost releases greenhouse gases, and those gases contribute to further warming."
Which of the following sentences most logically fills the [BLANK]?
- A. Permafrost regions are popular destinations for winter sports tourism.
- B. Trapped within this frozen ground is an enormous store of ancient organic matter — the partially decomposed remains of plants and animals accumulated over thousands of years.
- C. Most permafrost is located at high altitudes rather than high latitudes.
- D. Scientists first identified permafrost in the nineteenth century.
Show answer and solution
Answer: (B) Trapped within this frozen ground is an enormous store of ancient organic matter — the partially decomposed remains of plants and animals accumulated over thousands of years.
The sentence following the blank refers to 'this organic matter' becoming available to microbes — so the blank must introduce that organic matter. Option B does exactly this, setting up the rest of the passage's explanation of the carbon/methane feedback loop. A, C and D introduce unrelated or tangential facts that the following sentence does not build on.
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