Single correct option
The passage below is followed by a sentence that has been removed from it. From the given options, choose the one where the missing sentence best fits back into the passage.
[1] Antarctic ice cores allow scientists to reconstruct atmospheric composition going back roughly eight hundred thousand years, by analysing tiny bubbles of ancient air trapped as snow compacted into ice. [2] Each layer of ice corresponds to a period of snowfall, and the depth of a given layer, combined with known snowfall rates, lets researchers assign it an approximate date. [3] The oldest, deepest cores require drilling through more than three kilometres of ice, a task that can take several field seasons to complete safely. [4] What makes these records unusually valuable is that they capture not just temperature proxies but the actual chemical composition of the ancient atmosphere itself, rather than an indirect estimate of it.
Missing sentence: Confirming a layer's age independently, researchers cross-check this estimate against known volcanic eruptions, whose ash leaves a distinctive chemical signature at a precisely documented point in the historical record.
[1] Antarctic ice cores allow scientists to reconstruct atmospheric composition going back roughly eight hundred thousand years, by analysing tiny bubbles of ancient air trapped as snow compacted into ice. [2] Each layer of ice corresponds to a period of snowfall, and the depth of a given layer, combined with known snowfall rates, lets researchers assign it an approximate date. [3] The oldest, deepest cores require drilling through more than three kilometres of ice, a task that can take several field seasons to complete safely. [4] What makes these records unusually valuable is that they capture not just temperature proxies but the actual chemical composition of the ancient atmosphere itself, rather than an indirect estimate of it.
Missing sentence: Confirming a layer's age independently, researchers cross-check this estimate against known volcanic eruptions, whose ash leaves a distinctive chemical signature at a precisely documented point in the historical record.
- A. After [1]
- B. After [2]
- C. After [3]
- D. After [4]
Show answer and solution
Answer: (B) After [2]
The missing sentence begins 'Confirming a layer's age independently,' which requires an existing age-estimate to confirm — this connects directly to sentence [2]'s description of assigning an approximate date from depth and snowfall rate. Placing it after [2] lets the volcanic cross-check serve as a natural elaboration on how that dating is verified, before the passage shifts to drilling logistics [3] and then to the composition point [4]. It cannot precede [2], since there is no dating method yet to confirm.
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