Question 101·Medium·Rhetorical Synthesis
While researching a topic, a student has compiled the following notes:
• The atmospheric lifetime of carbon dioxide (CO₂) is roughly 300–1,000 years.
• The atmospheric lifetime of methane (CH₄) is about 12 years.
• Over a 20-year period, CH₄ has approximately 84 times the global-warming potential of CO₂.
• Current atmospheric concentrations of CO₂ greatly exceed those of CH₄.
The student wants to emphasize the difference in atmospheric lifetimes of the two greenhouse gases. Which choice most effectively uses relevant information from the notes to accomplish this goal?
For note-based rhetorical synthesis questions, first underline the exact goal in the question (for example, “emphasize the difference in atmospheric lifetimes”). Next, scan the notes and mark only the bullets that directly support that goal; ignore details about other aspects (like concentration or general effects) if they’re not requested. Then eliminate any answer choices that focus on irrelevant notes or mention only one of the items that need to be compared. Choose the option that uses the most relevant notes and clearly expresses the required relationship (such as a contrast or cause-and-effect) in a single, well-focused sentence.
Hints
Lock in on the exact writing goal
Underline the phrase “emphasize the difference in atmospheric lifetimes of the two greenhouse gases”. Any answer that doesn’t clearly compare how long both gases stay in the atmosphere should be suspicious.
Decide which notes actually help with that goal
Look back at the bullet points and ask: which ones talk specifically about atmospheric lifetime, and which ones are about something else like warming power or concentration?
Match an answer to both relevant notes
Among the choices, look for a sentence that (1) mentions both methane and carbon dioxide, and (2) clearly contrasts how long each one stays in the atmosphere using information from the relevant notes.
Step-by-step Explanation
Identify the task in the question
The question says the student wants to “emphasize the difference in atmospheric lifetimes of the two greenhouse gases.”
So the correct answer must:
- Talk about atmospheric lifetime (how long the gases stay in the atmosphere), and
- Clearly show a difference/comparison between both methane (CH₄) and carbon dioxide (CO₂).
Pick out the relevant notes
From the notes, find which bullets relate to atmospheric lifetime:
- Note 1: “The atmospheric lifetime of carbon dioxide (CO₂) is roughly 300–1,000 years.” → relevant
- Note 2: “The atmospheric lifetime of methane (CH₄) is about 12 years.” → relevant
- Note 3: “Over a 20-year period, CH₄ has approximately 84 times the global-warming potential of CO₂.” → about warming strength, not lifetime
- Note 4: “Current atmospheric concentrations of CO₂ greatly exceed those of CH₄.” → about concentration, not lifetime
To meet the goal, a good answer should mainly use Notes 1 and 2, not Notes 3 or 4.
Check which choices match the goal and notes
Now compare each answer choice to the goal and the relevant notes:
- Choice A talks about concentrations and being the most abundant greenhouse gas → uses Note 4, not about lifetime.
- Choice B mentions methane’s high warming power and briefly its short lifetime → uses Notes 2 and 3, but mainly focuses on warming power, and doesn’t state CO₂’s lifetime.
- Choice C compares how long methane and carbon dioxide stay in the atmosphere, with numbers for each → uses Notes 1 and 2 and clearly shows a contrast.
- Choice D says both gases trap heat and affect climate change → a general statement, not about lifetimes or their difference.
Select the option that best emphasizes the lifetime difference
Only one choice directly and clearly contrasts the atmospheric lifetimes of both gases using the relevant information from the notes (the 12-year lifetime for methane and the 300–1,000-year lifetime for carbon dioxide). That choice is:
C) While methane lingers in the atmosphere for only about 12 years, carbon dioxide can persist for centuries—anywhere from 300 to 1,000 years.