Ochre sea stars live in tidal pools along the shoreline of the Pacific Ocean. At night, they move to higher...
GMAT Information and Ideas : (Ideas) Questions
Ochre sea stars live in tidal pools along the shoreline of the Pacific Ocean. At night, they move to higher shore levels in search of prey. But scientists Corey Garza and Carlos Robles noticed that ochre sea stars stayed at lower levels at night after heavy rains. Garza and Robles hypothesized that a layer of fresh water formed by rainfall was a barrier to the sea stars. To test their hypothesis, the scientists did an experiment. They placed some sea stars in a climbable tank of seawater and other sea stars in a similar tank of seawater with a layer of fresh water on top. Then, the scientists watched the sea stars' behavior at night.
Which finding from the experiment, if true, would most directly support Garza and Robles's hypothesis?
None of the sea stars climbed to the tops of the tanks, but sea stars in the tank with only seawater moved around the bottom of the tank more than sea stars in the other tank did.
Sea stars in the tank with only seawater climbed to the top of the tank, but sea stars in the other tank stopped climbing just below the layer of fresh water.
Both groups of sea stars climbed to the tops of the tanks, but sea stars in the tank with only seawater climbed more slowly than sea stars in the other tank did.
Sea stars in the tank with only seawater mostly stayed near the bottom of the tank, but sea stars in the other tank climbed into the layer of fresh water.
Solution
Step 1: Decode and Map the Passage
Create Passage Analysis Table
| Text from Passage | Analysis |
|---|---|
| "Ochre sea stars live in tidal pools along the shoreline of the Pacific Ocean." |
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| "At night, they move to higher shore levels in search of prey." |
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| "But scientists Corey Garza and Carlos Robles noticed that ochre sea stars stayed at lower levels at night after heavy rains." |
|
| "Garza and Robles hypothesized that a layer of fresh water formed by rainfall was a barrier to the sea stars." |
|
| "To test their hypothesis, the scientists did an experiment." |
|
| "They placed some sea stars in a climbable tank of seawater and other sea stars in a similar tank of seawater with a layer of fresh water on top." |
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| "Then, the scientists watched the sea stars' behavior at night." |
|
Provide Passage Architecture & Core Elements
Main Point: Scientists tested whether fresh water layers from rainfall act as barriers preventing sea stars from their normal upward movement at night.
Argument Flow: The passage establishes normal sea star behavior (moving upward at night), presents an anomaly observed after rainfall (staying low), proposes a hypothesis to explain this anomaly (fresh water barrier), and describes a controlled experiment designed to test that hypothesis using two different tank conditions.
Step 2: Interpret the Question Precisely
What's being asked? Which experimental finding would most directly support Garza and Robles's hypothesis.
What type of answer do we need? Evidence that shows fresh water layers act as barriers to sea star movement.
Any limiting keywords? "Most directly support" - we need the finding that provides the clearest, most direct evidence for their hypothesis.
Question Characterization:
- Content Genre: Sciences
- Content Format: Text-only
- Question Type: Strengthen / Weaken
- Language Complexity: Accessible
Step 3: Prethink the Answer
- To support the hypothesis that fresh water layers act as barriers, we need evidence showing that sea stars behave differently when fresh water is present versus when it's not
- Specifically, we'd expect to see normal climbing behavior in the seawater-only tank but impaired or stopped climbing when sea stars encounter the fresh water layer
- The key contrast should be between the two experimental conditions, with the fresh water clearly acting as an obstacle
- So the right answer should show sea stars climbing normally in seawater but being stopped or deterred by the fresh water layer
None of the sea stars climbed to the tops of the tanks, but sea stars in the tank with only seawater moved around the bottom of the tank more than sea stars in the other tank did.
✗ Incorrect
- Claims neither group climbed to the tops, but seawater-only group moved around bottom more
- This doesn't demonstrate fresh water as a barrier - if both groups failed to climb, there's no evidence that fresh water specifically prevented climbing
Sea stars in the tank with only seawater climbed to the top of the tank, but sea stars in the other tank stopped climbing just below the layer of fresh water.
✓ Correct
- Shows seawater-only group climbed to the top while the other group stopped just below the fresh water layer
- This directly supports the barrier hypothesis - sea stars climb normally until they encounter fresh water, then stop
Both groups of sea stars climbed to the tops of the tanks, but sea stars in the tank with only seawater climbed more slowly than sea stars in the other tank did.
✗ Incorrect
- States both groups reached the tops but at different speeds
- If both groups successfully climbed through or past the fresh water, this contradicts the barrier hypothesis
Sea stars in the tank with only seawater mostly stayed near the bottom of the tank, but sea stars in the other tank climbed into the layer of fresh water.
✗ Incorrect
- Claims seawater-only group stayed near bottom while fresh water group climbed into the fresh water layer
- This is completely opposite to what the hypothesis predicts - it suggests fresh water attracts rather than repels sea stars