Question 108·Hard·Text Structure and Purpose
Over the last decade, field surveys have noted that several high-elevation lakes are “blueing” rather than “greening”—a shift in color associated with declining algal biomass. While some observers attribute this trend to reduced nutrient runoff brought about by improved land management, the timing of the change in remote catchments with no agriculture argues against that view. Nor can the shift be traced to diminished airborne dust alone: long-term deposition records show no synchronized decline. Instead, as Lina Ortega and colleagues contend, rapidly shrinking snowpacks are shortening the period when meltwater delivers nutrients, starving these lakes even as their waters become optically clearer.
Which choice best describes the overall structure of the text?
For text-structure questions, first break the passage into parts (usually by sentences) and write a 2–4 word label for what each part does (e.g., "state trend," "reject cause," "offer new cause"). Then scan the answer choices for the one whose sequence of actions matches your labels from start to finish, and quickly eliminate any choice that mentions things the passage never does (like methods, comparisons, or conclusions that aren’t there). This keeps you focused on overall organization rather than being distracted by familiar-sounding phrases.
Hints
Look at the job of each sentence
Briefly label each sentence: What does the first sentence mainly do? What about the second, the third, and the fourth? Think in terms of roles like observation, explanation, evidence, or conclusion.
Pay attention to signal words
Notice linking words such as "While," "Nor," and "Instead." These words tell you how each new idea relates to the previous one—for example, contrast, addition, or replacement.
Check for what the passage does not do
Ask yourself: Does the passage describe research methods? Does it say the change is unimportant? Does it mainly compare two locations? Eliminate any answer whose description includes something the passage never does.
Match your outline, not individual phrases
After you outline the passage, choose the answer whose full sequence of actions matches your outline from beginning to end, not just one part of it.
Step-by-step Explanation
Identify what happens in the first sentence
Read the first sentence: it describes field surveys over the last decade and notes that some high-elevation lakes are "blueing" instead of "greening," which is linked to declining algal biomass.
This sentence is introducing a recent ecological trend/observation—a change in lake color connected to a biological change.
See how the second and third sentences treat possible causes
The second sentence starts with "While some observers attribute this trend to reduced nutrient runoff..." and then explains why evidence from remote catchments "argues against that view." So it raises one explanation and then rejects it using timing evidence.
The third sentence starts with "Nor can the shift be traced to diminished airborne dust alone" and then cites long-term records that show no matching decline. This raises another possible cause and rejects it based on data as well.
Determine the role of the final sentence
The last sentence begins with "Instead" and then introduces Lina Ortega and colleagues' idea that shrinking snowpacks shorten the nutrient-delivery period, which starves the lakes even as they become clearer.
The word "Instead" signals a replacement explanation: after dismissing earlier possibilities, the author now presents a different cause and treats it as the supported one.
Match this structure to the answer choices
Summarize the structure: the passage (1) states an observed ecological change, (2) considers and rules out two proposed explanations, and (3) puts forward a different explanation as better supported.
Among the options, "It presents a recent ecological observation, rules out two common explanations for it, and endorses a third explanation as better supported." is the only one that fits this pattern exactly, so that is the correct answer.