For a command-of-evidence question about a causal explanation, separate the explanation into its required links and identify any competing explanation named in the passage. Then choose the finding that tests the full causal chain while ruling out the alternative; evidence showing only correlation, biological possibility, or one stage of the mechanism is less direct.
Hints
- Hint 1
Break the claim into parts
Identify where the water is said to originate, how it is said to reach the surface, and which plants are said to absorb it.
- Hint 2
Consider the alternative
Look for evidence that distinguishes underground water movement from the reduced water loss that cooler, darker conditions could cause.
- Hint 3
Prefer causal evidence
A direct test of the water’s source and route is stronger than a correlation between proximity to mature trees and juvenile hydration.
Step-by-step
- Step 1
Identify the proposed mechanism
Venn claims that mature trees move deep water into surface soil at night and that shallow-rooted juveniles then absorb this water.
- Step 2
Account for the competing explanation
Because shade could independently reduce juvenile water loss, the strongest evidence must control light and temperature rather than merely show that juveniles near mature trees contain more water.
- Step 3
Select the most direct evidence
Choice A establishes the water’s deep source, prevents juveniles from accessing that source directly, traces the water into surface soil and juvenile leaves, and controls canopy conditions. It therefore supports the complete proposed mechanism rather than only one part of it.