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, first briefly paraphrase the passage in your own words, focusing on what changes from the beginning to the end. Then, separate main ideas from supporting details: examples, numbers, or specific places are usually details, not the core point. Eliminate answers that focus only on one example, add information not in the passage, or shift the emphasis (for instance, from “what new tools reveal” to “how hard they are to use”). Choose the option that best captures the overall message and applies to the entire passage.
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
Notice that the Guatemala and Sahara cases are specific examples. Ask yourself: what overall point are these examples being used to support?
Check for scope and emphasis
For each answer choice, ask: does this idea apply to the whole passage, or only to one part? Also, does the passage actually emphasize what the choice emphasizes (like difficulties, speed, or one location)?
Step-by-step Explanation
Identify what the passage is mostly about
Focus on the first and last sentences, since they often frame the central idea.
- The first sentence describes old methods: walking surveys and test trenches that could miss sites hidden by forests or cities.
- The second and third sentences introduce new instruments on aircraft or satellites (LiDAR, radar) and give examples of what they’ve revealed in Guatemala and the Sahara.
- The last sentence says these findings have forced researchers to revise long-held assumptions about ancient societies.
So overall, the passage is about a shift in archaeology due to new technologies and how that shift has changed what archaeologists believe about the past.
Distinguish main idea from specific examples
Ask: which parts are supporting details, and which part describes the big overall point?
- The Guatemalan roads and temples and Saharan irrigation canals are examples of what the new tools can uncover.
- The central idea must cover all those examples and also the contrast between old and new methods.
- It should also include the impact: that these discoveries have changed archaeologists’ understanding.
Keep in mind: a correct central-idea answer is broad enough to summarize the whole passage, not just one example or side point.
Eliminate choices that are too narrow or off-topic
Now compare each answer choice to the overall idea you’ve identified.
- Choice A focuses only on LiDAR surveys in Guatemala and the roads and temples there. That’s just one example, not the whole point, and it ignores the Sahara example and the broader impact on understanding ancient societies.
- Choice B emphasizes that traditional methods are “slow and imprecise” and contrasts them with satellite imagery. The passage does say the old process could take years and miss sites, but it doesn’t focus on imprecision or criticize old methods as the main point; instead, it emphasizes what new tools have revealed and how that changes assumptions.
- Choice C claims that interpreting data from these instruments presents new challenges, especially in forests. The passage never talks about difficulties interpreting the data; it only describes successful discoveries.
Only one remaining choice accurately captures both the new technology and its big effect on archaeologists’ ideas.
Confirm the choice that matches the passage’s main point
The remaining option, Choice D, states that new remote-sensing tools enable archaeologists to identify and study previously hidden archaeological sites, fundamentally altering their understanding of the ancient world. This matches the passage’s description of aircraft- and satellite-based tools (LiDAR, radar) revealing hidden sites and forcing researchers to revise long-held assumptions, so D is the correct answer.