Question 41·Medium·Central Ideas and Details
Scientists studying ancient coral skeletons discovered that layers of varying density correspond to historical ocean temperature fluctuations. By drilling core samples from 400-year-old corals in the South Pacific, the team measured isotopic ratios that revealed seasonal and decadal climate patterns. When they compared these data with modern instrumental records, the two sets showed remarkable agreement, confirming that coral archives are reliable indicators of past sea-surface temperatures. The researchers contend that analyzing even older coral formations could yield valuable insights into long-term climate cycles.
Which choice best states the main idea of the text?
For SAT main idea questions, read the entire passage once, then especially reread the first and last sentences, since they often frame the central point. Ask yourself: "What did the author mainly want to show or argue?"—focusing on the overall conclusion rather than specific methods or details. Then eliminate choices that either introduce ideas not mentioned, exaggerate or distort what the passage says, or focus on side details instead of the central conclusion, and choose the option that captures both the key finding and any stated implications or next steps.
Hints
Focus on the beginning and the end
Reread the first and last sentences. What do they say about what corals can tell us and what the researchers want to do next?
Separate details from the main point
Some sentences describe how the scientists did the study (drilling, isotopic ratios, comparisons). Ask: what overall conclusion do these details support?
Watch out for new ideas not in the text
Be careful with choices that mention threats to reefs, big long-term changes, or which data source is better if those ideas are not clearly stated in the passage.
Step-by-step Explanation
Identify what the scientists did and found
First, focus on the actions and findings described:
- They studied ancient coral skeletons.
- They discovered that density layers match historical ocean temperature changes.
- They measured isotopic ratios and saw seasonal and decadal climate patterns.
- When they compared coral data with modern instrumental records, the two agreed, which confirmed the reliability of coral as an indicator of past sea-surface temperatures. These points show the passage is mainly about using corals as records of past ocean temperatures.
Notice the author’s concluding claim
Look closely at the last sentence: "The researchers contend that analyzing even older coral formations could yield valuable insights into long-term climate cycles." This adds a forward-looking idea:
- Not just that coral records are reliable now
- But also that future study of older corals could reveal long-term climate cycles. So the main idea must include both the reliability of coral records and the potential value of expanding this research.
Eliminate choices that miss key aspects of the passage
Now test each option against the passage:
- Remove any choice that focuses on threats to reefs or research methods, because the passage never criticizes the drilling.
- Remove choices that claim large changes over 400 years; that level of change is not stated.
- Remove choices that say instrumental records are better or should be primary, because the passage only says they agree with coral data, not that one is superior. The remaining choice will be the one that states that coral records are accurate indicators of past temperatures and that studying older corals could help us understand long-term climate patterns.
Select the answer that matches both the findings and the future implications
The best answer is C) Ancient coral skeletons can serve as accurate records of past ocean temperatures, and expanding this research could improve understanding of long-term climate patterns.
This directly matches the passage’s two main points:
- Corals are confirmed to be reliable indicators of past sea-surface temperatures.
- Studying even older corals could give valuable insights into long-term climate cycles.