A finding question asking you to support one explanation and weaken another tests which cause the evidence favors. Read the question to see which explanation it asks you to favor, then put both explanations in a few words. Predict what would tell them apart, and check every result the claim covers. A finding about only one result leaves part of the claim untested. Evidence that fits both explanations does less to settle the question than evidence that favors one over the other.
Hints
- Hint 1
Read the passage’s last sentence and put the selection-effect explanation in a few words. The question asks you to support that explanation, even though the researchers argue against it.
- Hint 2
Find the two results being explained in the sentence about the season’s end and storm surges. What did the researchers measure in each case?
- Hint 3
Compare what the rival explanations predict about mixtures and single variants. What would test whether a variant’s own performance can account for a mixture’s advantage?
Step-by-step
Compare mixtures with one variant alone
Step 1Put the rival claim in a few words
The last sentence describes a selection effect: a mixture does well because it includes an unusually effective variant. Which explanation should the evidence favor? We’re asked to support the selection-effect explanation, so we need one variant to explain the mixture’s success, rather than several variants helping one another.
- Step 2
Find the comparison that separates the causes
The reported advantages are more aboveground biomass and retained more sediment, both measured against the average monoculture, a plot with one variant. What comparison would tell the explanations apart? The average alone can't distinguish the causes, because one strong variant can lift a mixture above that average. Compare the mixture with that variant grown alone: matching it could support selection effect, while beating it would mean its individual performance can't explain the gain.
- Step 3
Remove findings that miss the rival claim
Which findings don't support that explanation? Mixtures lacking Delta beat Delta alone at retaining sediment, so Delta can't explain that gain. Every mixture produced more biomass than its variants alone, so no variant’s individual performance explains that gain. The sharply different root depths finding links sediment retention to differences among variants at equal biomass, favoring variants helping one another.
- Step 4
Test both gains against Delta alone
The finding only when variant Delta was present ties the mixture’s advantage to Delta. Those mixtures also perform about the same as Delta monocultures in both biomass and sediment retention, so Delta alone can account for both gains. Those mixtures don't show a gain that no one variant can achieve alone, which weakens the researchers’ interpretation. Choice A.