A question about strengthening a study's conclusion tests whether new evidence supports the proposed cause over another cause. Read the conclusion, then put the cause and effect in a few words. If the study combines different groups, ask what changed besides the proposed cause and how that could affect the result. Support must show the expected effect and make another explanation less likely. Evidence that repeats the overall pattern may leave the cause uncertain.
Hints
- Hint 1
Read the proposal about atmospheric carbon dioxide alongside the sentence about the fossil layers. Separate the measured change from its proposed cause.
- Hint 2
Compare species R and S in the controlled-growth studies. What difference between the species could affect the fossil average?
- Hint 3
The controlled-growth studies show that both species respond to carbon dioxide, but by different amounts. Ask what evidence could distinguish a carbon dioxide change from a change in the species mix.
Step-by-step
Test the two possible causes
Step 1Find the cause being claimed
The average stomatal index of fossil birch leaves declines in successively younger layers, and the researchers take that as evidence of rising carbon dioxide. The pooled index combines leaves from both species. What does the blank ask us to strengthen? It's the proposed cause of the decline, not the decline itself.
- Step 2
Find the competing explanation
Could the shift from mainly R to mainly S produce that decline without a carbon dioxide increase? The growth studies say species S has a lower index than species R at every tested concentration. So more S could lower the average on its own, leaving two possible causes for the pooled trend. We need evidence that rising carbon dioxide mattered beyond that species shift.
- Step 3
Use each species's measured response
Rising carbon dioxide has a known effect: each develops a lower stomatal index when carbon dioxide increases. But how much should the index fall? Since the size of each species's response differs, R and S need checks against their own measured responses. A fall in the fossil index should match what increased carbon dioxide does to that particular species.
- Step 4
Hold the species fixed
What comparison avoids the shift in species composition? The finding beginning find, within each species compares R with R and S with S across layers, so replacing one species with another can't explain those declines. Matching each decline to that species's measured carbon dioxide response then connects the fossil change to the proposed cause. Choice A.