Question 21·Hard·Central Ideas and Details
For much of the twentieth century, archaeologists located ruins mainly by surveying the ground on foot and digging test trenches, a process that could take years and still miss important sites concealed by forests or urban development. Over the past two decades, however, instruments that collect data from aircraft or satellites have revolutionized the field. High-resolution LiDAR (light detection and ranging) systems, which can penetrate dense vegetation, have uncovered networks of roads and temples in the Guatemalan lowlands, while radar images taken from orbit have traced buried irrigation canals beneath Saharan sands. These and similar findings have forced researchers to revise long-held assumptions about the scale and complexity of ancient societies.
Which choice best states the central idea of the text?
For central-idea questions, quickly summarize the passage in one sentence by linking the setup (old approach/problem) to the conclusion (new development/result). Treat specific places and examples as supporting details, not the main point. Then choose the option that matches the passage’s overall scope and avoids adding claims the text doesn’t make or narrowing the focus to a single example or minor detail.
Hints
Use the beginning and end of the passage
Reread the first and last sentences. What problem is introduced at the start, and what broad result or conclusion is given at the end?
Look beyond individual examples
The Guatemala and Sahara details are examples. Ask what single message those examples are meant to prove.
Check for what’s missing or added
A correct central-idea choice shouldn’t add claims the passage never makes, and it shouldn’t omit the passage’s concluding takeaway.
Step-by-step Explanation
Pinpoint the passage’s overall claim
The passage contrasts older archaeology methods (walking surveys and test trenches that can miss hidden sites) with newer remote-sensing tools on aircraft or satellites. It then explains that these tools have uncovered previously hidden features and that the resulting discoveries have forced researchers to revise long-held assumptions about ancient societies.
Define what a central-idea answer must include
A central-idea answer should:
- Be broad enough to cover both examples (Guatemala and the Sahara).
- Emphasize the new remote-sensing tools (not just one tool or one place).
- Include the impact: the discoveries are changing what archaeologists think about ancient societies.
Test each choice for scope and accuracy
- Choice A matches part of the passage (remote sensing finds hidden sites) but leaves out the key conclusion that these findings change researchers’ understanding.
- Choice B goes beyond the passage by claiming archaeologists can determine society size without excavation and that traditional methods are becoming unnecessary—neither claim is stated.
- Choice C focuses on selecting tools for different environments, which is a supporting detail at best; the passage’s main point is the broader revolution in discovery and interpretation.
That leaves the option that captures both the discovery power of the tools and their effect on long-held assumptions.
Select the choice that matches the passage’s main point
Choice D best states the central idea because it summarizes the role of remote-sensing tools in revealing hidden sites and explains that these discoveries are reshaping archaeologists’ understanding of the ancient world.