Question 161·Medium·Central Ideas and Details
For decades, botanists assumed that a rainforest tree’s root zone extended no deeper than about two meters below the forest floor. Recently, however, a team led by ecologist Priya Raman installed moisture and nutrient sensors in boreholes reaching nearly ten meters down at several Amazonian sites. The data revealed that during dry months the trees draw half of their water from depths greater than five meters and absorb dissolved minerals found only in those lower layers. Raman’s team therefore concludes that deep roots play a critical role in helping mature rainforest trees survive seasonal drought.
Which choice best states the main idea of the text?
For main idea questions in SAT Reading & Writing, first read the entire passage, then briefly restate in your own words what the author is mainly trying to show or prove. Pay special attention to the first sentence (setup) and the last sentence (conclusion), and then eliminate answer choices that (1) add new claims not mentioned, (2) focus on a minor detail instead of the overall point, or (3) contradict the passage. Choose the option that best matches your own short summary of the passage’s central message.
Hints
Locate the key sentence
Look closely at the first and last sentences of the passage. What long-held assumption is described at the beginning, and what conclusion do the scientists reach at the end?
Focus on conclusions, not details
The question asks for the main idea, not a small detail. Ask yourself: What overall claim are the scientists making about rainforest tree roots based on their data?
Watch out for extra or opposite information
Eliminate any choices that talk about things the passage never mentions (like growth rates or changes in drought severity) or that say the opposite of what the data show about root depth.
Step-by-step Explanation
Identify the general topic and purpose
Read the passage to see what it is mainly about.
The passage discusses rainforest trees, specifically how deep their roots go, and describes an experiment using moisture and nutrient sensors in deep boreholes. The purpose is to show what the new data reveal about tree roots and drought survival.
Find the main conclusion
On SAT main idea questions, the main conclusion is often in the final sentence or in a summary statement.
The last sentence says: "Raman’s team therefore concludes that deep roots play a critical role in helping mature rainforest trees survive seasonal drought." This is the key takeaway. The experiment with boreholes and sensors leads to this conclusion about deep roots and their role in getting resources during dry periods.
Test each answer against the passage
Now compare each answer choice to the passage:
- One choice claims the trees grow faster because deeper soil has more nutrients. The passage never mentions growth speed or nutrient concentrations; it only says minerals are found in lower layers.
- Another says drought conditions are more severe than believed. The passage does mention seasonal drought but does not compare its severity to earlier beliefs.
- Another says roots remain close to the surface even during dry periods, which directly opposes the data that trees draw half their water from depths greater than five meters.
- One remaining choice matches the idea that the new measurements show mature rainforest trees use deep roots to access water and minerals during dry months.
Select the answer that matches the main idea
The choice that best states the main idea is the one that combines the new measurement method with the key conclusion about deep roots and resource uptake:
Correct answer: B) A new sensing technique shows that mature rainforest trees rely on unexpectedly deep roots to obtain water and minerals.