Question 135·Hard·Inferences
During a decades-long study of the Silvertop Glacier in the Cascade Range, glaciologist Luis Moreno compared annual layers of ice cores with historical records of airborne pollutants. He observed that layers corresponding to years with major volcanic eruptions contained heavy concentrations of sulfate but little black carbon, while layers from the mid-20th-century industrial boom showed the opposite pattern—high black carbon and relatively low sulfate. Unexpectedly, several late-19th-century layers exhibited elevated levels of both sulfate and black carbon. Consulting meteorological archives, Moreno found that these years immediately followed minor volcanic eruptions and also saw extensive regional forest fires. Moreno therefore proposed that ______
Which choice most logically completes the text?
For inference sentence-completion questions, first restate the passage’s established relationships (which events match which effects). Then use any signal word like “therefore” to predict a conclusion that directly synthesizes the provided evidence. Eliminate choices that ignore one key piece of evidence, swap the established relationships, or add an unsupported new cause.
Hints
Map pollutants to sources
From the first part of the text, decide which source best matches sulfate and which best matches black carbon.
Use the archive findings
The meteorological archives add two facts (minor eruptions and forest fires). The correct completion should use both facts, not treat one as irrelevant.
Avoid unsupported new explanations
Be cautious of choices that introduce a new cause (like measurement error/contamination) that the passage never suggests.
Step-by-step Explanation
Identify the key pattern in the evidence
Moreno’s comparisons show a consistent pattern:
- Major volcanic eruption years: heavy sulfate, little black carbon.
- Mid-20th-century industrial boom: high black carbon, relatively low sulfate.
- Late-19th-century layers: both sulfate and black carbon are elevated.
So sulfate tracks with volcanic activity, while black carbon tracks with burning/industrial sources.
Use the additional records for the late-19th-century years
For the late-19th-century years with both pollutants, Moreno finds two relevant coinciding events:
- They followed minor volcanic eruptions.
- They also saw extensive regional forest fires.
Those records supply a plausible source for each pollutant.
Match each event to the pollutant it best explains
From the earlier pattern:
- Volcanic eruptions best explain sulfate.
- Widespread burning (like forest fires) best explains black carbon.
So the conclusion should combine both causes rather than attributing both pollutants to only one event type.
Select the choice that states that combined explanation
The best completion is the one that says the minor eruptions account for the sulfate and the forest fires account for the black carbon.
The correct answer is: "small volcanic eruptions in the late 19th century injected sulfate into the atmosphere, and extensive regional forest fires around the same time added substantial black carbon."