For a command-of-evidence question asking what would undermine a claim, identify every part of the claim and any alternative explanation given in the passage. Then choose the finding that most directly tests the disputed relationship. Evidence showing that the same outcome occurs when the proposed cause is removed is generally stronger than evidence merely showing that an alternative cause is present.
Hints
- Hint 1
Focus on the causal pathway
The claim concerns how the carbon reached the seedlings, not simply whether carbon moved between plants.
- Hint 2
Consider the alternative explanation
The passage identifies soil water as another possible route. Look for a finding that distinguishes this route from transfer through fungal threads.
- Hint 3
Test whether the network is necessary
Ask what would happen if the proposed fungal connection were physically unavailable but the alternative route remained open.
Step-by-step
- Step 1
Identify the claim
The researchers claim not merely that carbon moved from birches to seedlings but that most of it moved specifically through shared fungal threads rather than through soil.
- Step 2
Look for a test of the proposed pathway
The strongest weakening evidence would prevent fungal threads from reaching some seedlings while leaving the alternative soil-water pathway open. If those seedlings still became equally enriched, the fungal pathway would not be necessary to explain the observation.
- Step 3
Select the decisive finding
Choice C provides exactly that comparison: seedlings became just as enriched even when membranes blocked fungal threads. Therefore, the answer is In a replication, fir seedlings behind membranes that blocked fungal threads but admitted water and dissolved carbon became as enriched as unshielded seedlings.