A finding question that asks you to undermine a claim tests whether evidence weakens the researchers' explanation. Read the claim and put it in a few words, including which cause it favors and how much that cause explains. Then predict what would weaken the link: the cause without the effect, the effect without the cause, or evidence for another cause. Test each finding against that prediction. A finding can share the topic without showing why the effect happened.
Hints
- Hint 1
Look at how much carbon the underlined claim assigns to each route. The question asks you to challenge that explanation, not whether carbon-13 reached the seedlings.
- Hint 2
Separate the observation from its cause. What does carbon-13 enrichment tell you, and what does it leave uncertain about the route?
- Hint 3
How could you compare the two routes to test which one carried most of the carbon-13 into the seedlings?
Step-by-step
Separate the two routes
Step 1Distinguish the result from the claim
The fir seedlings later became enriched in carbon-13, so labeled carbon reached them. What does the underlined claim add? It says most of the carbon-13 entering the fir seedlings traveled through shared fungal threads, so the question asks us to challenge the main route the carbon took, not the transfer itself.
- Step 2
Predict what losing the main route would do
Could the seedlings become enriched without fungal transfer? The passage says carbon could travel through soil water before being absorbed by the seedlings, so finding labeled carbon doesn't tell us which route carried it. Let's predict a result: if fungi carried most of the carbon, removing that route should greatly reduce enrichment while the soil route stays open. The effect without the suggested cause would weaken that explanation.
- Step 3
Test what the findings show
Greater carbon-13 enrichment with the same fungal taxon supports the claimed route through fungal threads shared with the birches. Does carbon reaching nearby mature birches undermine that route? No, because it tracks carbon moving from seedlings to birches. Soil-water samples show labeled carbon was present, but don't show how much entered the seedlings through water.
- Step 4
Read the membrane test
In the replication, membranes blocked fungal threads but let water and dissolved carbon through, so the shielded seedlings had no fungal path from the birches. Yet they became as enriched as unshielded seedlings. The soil route can produce the same enrichment without fungal access, which undermines the claim that most of the carbon-13 traveled through shared threads. Choice C.