While researching green-roof performance, a student has taken the following notes:
- Researchers monitored stormwater runoff from 36 buildings over three years.
- The green roofs had an 8-, 12-, or 16-centimeter layer of soil-like growing medium; conventional roofs served as comparisons.
- Over the full monitoring period, every green roof reduced total runoff relative to conventional roofs.
- During moderate storms, increasing the depth from 8 to 12 centimeters substantially improved water retention, while increasing it from 12 to 16 centimeters produced little additional improvement.
- During the heaviest 10 percent of storms, all three green-roof depths became saturated early, and researchers found no measurable differences among them in peak runoff rate.
- The study did not determine whether total runoff during the heaviest storms differed among the three green-roof depths.
- The 16-centimeter roofs supported more plant species but imposed greater structural and installation costs.
The student wants to write a one-sentence overview of the study's main findings about runoff without claiming more than the study established. Which choice most effectively uses relevant information from the notes to accomplish this goal?
For rhetorical synthesis questions involving several measured outcomes, label each finding by its precise scope, condition, and metric before comparing the choices. Eliminate options that turn a limited result into a universal conclusion, combine distinct measures as though they were interchangeable, or add an overall judgment that the notes do not support.
Hints
Track the scope of each result
Determine whether each note applies to the full monitoring period, moderate storms, or only the heaviest storms.
Distinguish the runoff measures
A finding about peak runoff rate does not automatically establish the same conclusion about total runoff or every other runoff-related outcome.
Avoid unsupported evaluations
Be cautious of a choice that declares one roof depth best overall when the notes report several outcomes but do not make that overall ranking.
Step-by-step Explanation
Identify the requested overview
The sentence should summarize the main findings about runoff and avoid extending the results beyond what the researchers measured.
Separate the findings by measure and storm type
The notes establish three distinct points:
- Across the full monitoring period, all green roofs reduced total runoff relative to conventional roofs.
- In moderate storms, additional growing-medium depth improved retention, though the gain beyond 12 centimeters was small.
- In the heaviest storms, depth did not affect peak runoff rate. The notes explicitly say that total runoff by depth was not determined for those storms.
Evaluate the scope of each choice
Choice A incorrectly extends the benefits of greater depth to all storm conditions and claims that depth reduced peak runoff rates despite the finding that those rates did not differ in the heaviest storms. Choice B makes an unsupported judgment that one depth was most effective overall and shifts attention to cost. Choice C accurately mentions the shared reduction in total runoff but overgeneralizes the peak-rate result into a claim that depth offered no runoff-related benefit of any kind during the heaviest storms. Choice D preserves the distinctions among total runoff, retention in moderate storms, and peak runoff rate in the heaviest storms.
Select the precise synthesis
Choice D is the only option that includes the principal runoff findings while maintaining the limits of the evidence.