Question 33·Hard·Text Structure and Purpose
Until the 1970s, most astronomers accepted that the visible material in a galaxy accounted for nearly all of its mass. Working at the Carnegie Institution, astrophysicist Vera Rubin tested this assumption by measuring the rotation speeds of stars in the outer regions of spiral galaxies. Contrary to expectations, she found that these stars orbited just as rapidly as stars near the galactic centers, suggesting the presence of unseen mass exerting gravitational influence. Rubin's observations provided some of the earliest and most persuasive evidence for what is now called “dark matter,” an idea that has reshaped modern cosmology.
Which choice best describes the overall structure of the text?
For "overall structure" questions, ignore details and instead ask, “What is the function of the beginning, middle, and end of this passage?” Summarize each part in a few words (e.g., old idea → experiment → impact), then match that pattern to the answer choices. Watch for tempting choices that keep the same topic words but subtly change the order (switching what comes first) or change the relationship (results supporting the belief vs. challenging it).
Hints
Look at the first sentence
Ask yourself: is the first sentence telling a story from someone’s life, defining a term, describing an instrument, or stating a belief that many scientists used to have?
Examine the middle of the paragraph
Focus on the sentences about Vera Rubin’s work. Are they explaining her entire career, giving a word’s history, describing a device, or showing how she tested something?
Focus on the final sentence
What is the main job of the last sentence—does it talk about criticisms, word origins, technical features, or what Rubin’s findings meant for science overall?
Match the pattern, not the topic words
Don’t be distracted by words like "dark matter" or "cosmology." Instead, think about the pattern: first part, middle part, last part—and pick the option whose three-part description fits that pattern.
Step-by-step Explanation
Identify what each part of the passage is doing
Break the paragraph into parts:
- Sentence 1: States a commonly held belief/assumption about galactic mass.
- Sentences 2–3: Describe Rubin’s measurements and the surprising finding that contradicts the earlier assumption.
- Sentence 4: Explains the broader importance of the finding (early evidence for dark matter that reshaped cosmology).
Summarize the structure in your own words
The text follows this sequence:
- old, widely accepted idea → 2) observations/experiment that challenges it → 3) why that challenge matters.
Match the summarized structure to the best answer choice
Compare the passage’s sequence to each option:
- Choice 1 says the observations ultimately supported the belief, but the passage says Rubin’s findings were contrary to expectations.
- Choice 2 puts Rubin’s measurements before the earlier assumption, but the passage starts with the assumption first.
- Choice 3 describes a disagreement/debate being resolved; the passage instead describes a widely held assumption being challenged by evidence.
- Choice 4 matches exactly: belief → experiment/observations that challenge it → significance.
Therefore, the best answer is: It introduces a widely held belief, presents an experiment that challenged the belief, and explains the significance of the resulting discovery.