Question 35·Medium·Central Ideas and Details
In the forests of eastern Canada, ecologist Dana Mills and colleagues have been tracking the advance of the emerald ash borer, an invasive beetle that kills ash trees. By comparing tree-ring damage patterns in ash trunks at sites located along railway lines with patterns at sites far from any rails, the team discovered nearly identical infestation timelines at rail-adjacent sites but much slower spread elsewhere. Mills argues that freight trains unintentionally speed the beetle’s movement by providing sheltered niches inside cargo cars, allowing the insect to leapfrog great distances. She concludes that rail transport is a previously overlooked driver of the pest’s expansion across the region.
Which choice best states the main idea of the text?
For main-idea questions, first quickly restate the passage in your own words in one short sentence, focusing on what the author concludes or claims, not on specific methods or side details. Then scan the final 1–2 sentences, which often contain the explicit conclusion, and use that to refine your summary. Eliminate choices that (1) focus only on a narrow detail (like a research method), (2) introduce information or plans not mentioned in the passage, or (3) contradict the passage’s emphasis. Finally, pick the choice that best matches your overall summary of the passage’s main point.
Hints
Focus on the big picture, not small details
Ask yourself: If I had to explain this paragraph in one sentence, what would I say? Don’t get stuck on individual methods or future plans; think about the overall point the author is making.
Pay extra attention to the end of the passage
Main-idea questions are often answered by the author’s conclusion, which is usually found in the last one or two sentences. Reread those sentences and think about how they relate to the earlier evidence.
Check each choice against the whole paragraph
For each answer, ask: Does this cover all the key parts (the research, the pattern found, and the conclusion), or does it only mention one smaller detail?
Watch out for ideas that never appear in the text
If an answer choice introduces a place, plan, or focus that the passage never mentions, that choice cannot be the main idea.
Step-by-step Explanation
Identify what the question is asking
The question asks for the main idea of the text. That means you need the answer that sums up the overall point of the whole paragraph, not a small detail or a side note.
Summarize the paragraph in your own words
Briefly restate the passage:
- Researchers are tracking an invasive beetle in eastern Canada.
- They compare tree-ring damage near railway lines to areas far from rails.
- They find much faster, similar infestation timelines along the rails, and slower spread elsewhere.
- Mills then argues why this is happening and states a conclusion about what is driving the beetle’s spread.
Keep in mind that the main idea usually matches the author’s conclusion or claim that ties all the details together.
Locate the author’s conclusion
Look carefully at the last two sentences:
- Mills argues that freight trains help the beetle move by giving it sheltered spots inside cargo cars so it can "leapfrog" long distances.
- The final sentence says she concludes that rail transport is a previously overlooked driver of the pest’s expansion across the region.
This conclusion explains why the earlier evidence about rail-adjacent vs. remote sites matters and pulls the whole paragraph together.
Match the conclusion to the best answer choice
Now compare that conclusion to the choices:
- The correct choice will capture the idea that the beetle’s rapid spread across eastern Canada is strongly connected to, and sped up by, rail transport, rather than just spreading slowly on its own.
- Choice D states this idea, so D) The emerald ash borer's rapid spread in eastern Canada is likely facilitated by rail transport rather than by natural dispersal alone is the best statement of the main idea.