Question 3·Hard·Central Ideas and Details
Until recently, geologists gauged the pace of coastal erosion mainly by comparing maps and shoreline surveys recorded decades apart. The advent of high-resolution satellite imagery, however, allows scientists to trace shoreline changes annually, revealing variations that long-term averages hide. One study of Alaska’s north coast demonstrated that, while the coastline receded an average of two meters per year between 1950 and 2000, annual retreat rates during the 2010s oscillated between less than one meter and more than six meters. Such findings warn against assuming that the past half-century’s modest rate will persist. They also highlight the necessity of combining historical records with contemporary observations to obtain a fuller picture of how quickly—and unpredictably—the land is yielding to the sea.
Which choice best states the main idea of the passage?
For main idea questions, quickly summarize the passage in your own words after reading, then focus especially on the first and last sentences, which often frame and restate the central point. When testing the answer choices, eliminate any option that (1) focuses too narrowly on one example or detail, (2) adds strong claims (like “only,” “obsolete,” “always,” “steadily”) that the passage doesn’t support, or (3) contradicts a key statement in the text; the correct choice will match both the overall message and the author’s explicit conclusions.
Hints
Look at the beginning and the end
Reread the first and last sentences of the passage. What change in scientists’ methods is described, and what “necessity” do the findings highlight?
Interpret the Alaska example
Ask yourself: Does the Alaska data show a smooth, steady increase in erosion, or does it show big year-to-year swings? How does that relate to the idea of a single long-term average?
Check what each choice emphasizes
For each answer, ask: Does it (1) talk about both older records and new satellite data, and (2) say something about why long-term averages or assumptions about past rates might be a problem?
Step-by-step Explanation
Identify how the passage is organized
Read the passage and notice its structure:
- Sentence 1: Describes the old method (maps and surveys decades apart).
- Sentence 2: Introduces the new method (high-resolution satellite imagery tracing changes yearly) and says it reveals variations that long-term averages hide.
- Sentences 3–4: Gives an example from Alaska showing specific numbers and big swings in annual erosion.
- Final sentence: States what these findings “highlight” and why they matter.
This structure tells you the passage is not just about Alaska or satellites in isolation, but about what the newer data changes in our understanding of erosion.
Focus on the example and what it shows
Look closely at the example of Alaska’s north coast:
- Between 1950 and 2000: the average retreat is two meters per year.
- In the 2010s: annual rates “oscillated between less than one meter and more than six meters.”
So, the key idea from the example is large year-to-year variation in erosion rates that a single long-term average number does not show.
Use the author’s conclusion to find the central idea
Now focus on the last two sentences, which usually carry the main point:
- “Such findings warn against assuming that the past half-century’s modest rate will persist.” → We should not assume the past average rate will continue.
- “They also highlight the necessity of combining historical records with contemporary observations…” → The author stresses we need both old records and new satellite data to understand how quickly and unpredictably the coast is changing.
So the main idea must involve (1) not trusting long-term averages by themselves and (2) the importance of combining old and new data.
Match the main idea to the best answer choice
Now compare each choice to what you found:
- Any correct answer must mention both types of data (historical and satellite, or old and new) and the problem with relying on long-term averages because erosion fluctuates.
- Choices that say historical maps are “obsolete,” that current erosion “now exceeds six meters per year,” or that erosion has accelerated “steadily” ignore the passage’s emphasis on combining data and on variability.
Choice D — “Incorporating modern satellite data with older records reveals that coastal erosion rates can fluctuate widely, so relying solely on long-term averages is misleading.” — matches exactly: it mentions combining data sources and explains that long-term averages hide big fluctuations, which is the passage’s main idea.