Question 101·200 Super-Hard SAT Reading Questions·Information and Ideas

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Microbiologists investigating competition between two bacterial strains first grew strain P in a standard liquid medium. They then filtered out all strain P cells and introduced equal numbers of strain R cells into the resulting P-conditioned medium. For one treatment, the researchers restored glucose and nitrogen to the concentrations found in fresh medium. For another, they treated the medium with activated carbon, which traps many extracellular organic compounds but cannot be consumed by strain R. The researchers also treated fresh medium with activated carbon as a control.

Growth of Strain R under Different Conditions

Medium and treatment	Mean increase in optical density
Fresh medium	0.96
P-conditioned medium	0.29
P-conditioned medium with nutrients restored	0.33
P-conditioned medium treated with activated carbon	0.88
Fresh medium treated with activated carbon	0.92

The researchers concluded that strain P suppresses strain R primarily by releasing an inhibitory compound rather than by consuming nutrients.

Which choice most effectively describes data in the table that support the researchers’ conclusion?

For command-of-evidence questions involving experiments, first translate the claim into the competing explanations being tested. Then identify which treatment isolates each explanation and use any control condition to rule out an effect caused by the treatment itself. Choose the option that addresses the complete claim, not one that merely reports a relevant data pattern.

Hints

Hint 1
Separate the hypotheses
Determine which treatment tests nutrient depletion and which treatment targets compounds that may have been released into the medium.
Hint 2
Measure recovery
Compare how close strain R’s growth under each treatment comes to its growth in untreated fresh medium.
Hint 3
Check the control
Use the fresh-medium treatment to determine whether activated carbon itself substantially changes strain R’s growth.

Step-by-step

Step 1
Identify the competing explanations
The researchers are distinguishing between nutrient depletion and the release of an inhibitory compound. Evidence for the conclusion must address both possibilities.
Step 2
Compare the targeted treatments
Restoring nutrients increased growth only from 0.29 to 0.33, so depleted nutrients do not adequately explain the suppression. Activated carbon, which traps extracellular organic compounds, increased growth to 0.88.
Step 3
Use the control
Activated carbon changed growth in fresh medium only slightly, from 0.96 to 0.92. Thus, the strong recovery in conditioned medium is best explained by the removal of a compound released by strain P.

Question 101·200 Super-Hard SAT Reading Questions·Information and Ideas

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