An SAT Reading & Writing micro-topic under Text Structure and Purpose (Craft and Structure). Free to read — no account needed.
When a question asks for the function of an underlined sentence, it wants the sentence's job in the passage, not a summary of what it says. A single sentence can do many different jobs, so the answer describes the role it plays. Here are two examples, with the sentence in question underlined.
Consider this passage. "For centuries, sailors steered by the stars. The Polynesians memorized the rising points of dozens of stars to guide their canoes across thousands of miles of open ocean. This skill was passed down through generations of expert navigators." The underlined sentence gives a specific case of the general claim before it, so its function is to provide an example.
Now consider a different passage. "Everyone assumed the old bridge was perfectly safe. A recent inspection, however, revealed deep cracks hidden inside its main supports. The city closed the bridge the very next day." This time the underlined sentence turns against the assumption before it, so its function is to introduce a contrast.
Common jobs are worth recognizing: a sentence may introduce a claim, give an example, add a supporting detail, offer a contrast, or draw a conclusion. Name the job in your own words first, then pick the matching answer. Eliminate any choice that names the wrong job or misstates what the sentence actually says.
Worked examples
Most people believe that adding lanes to a highway will reduce traffic jams. In reality, wider highways often draw so many extra drivers that congestion soon returns to its old levels. Urban planners call this surprising effect "induced demand." What is the function of the underlined sentence? (A) It supports the common belief stated in the first sentence. (B) It presents a contrast that challenges the common belief. The underlined sentence pushes back against the first sentence's assumption by showing the opposite result. So (B) is correct: its function is to offer a contrast.
A small bakery tested three different recipes, yet every loaf came out dense and flat. The owner eventually discovered that the yeast had expired months earlier. Clearly, the failed loaves were caused by the dead yeast, not by the recipes themselves. What is the function of the underlined sentence? (A) It draws a conclusion from the evidence given before it. (B) It introduces a brand-new problem the bakery faced. The underlined sentence uses the expired-yeast fact to reach a verdict about the cause. So (A) is correct: its function is to draw a conclusion.
Bioluminescence, the ability of living things to produce their own light, is common in the deep ocean. Anglerfish, for example, dangle a glowing lure to draw prey straight toward their mouths. This eerie light gives them a powerful advantage in the pitch-black depths. What is the function of the underlined sentence? (A) It gives a specific example of the general idea in the first sentence. (B) It argues against the idea stated in the first sentence. The underlined sentence illustrates the general point about bioluminescence with one concrete creature. So (A) is correct: its function is to give an example.
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