Question 28·200 Super-Hard SAT Reading Questions·Information and Ideas
Materials scientist Amara Okafor and colleagues compared uncoated steel samples with samples covered by an ultrathin water-resistant coating. The researchers exposed both types of sample to alternating periods of saltwater immersion and dry air. Two regimens provided the same total immersion time and the same total drying time, but one switched conditions every 12 hours, whereas the other switched every 2 hours. Under the 12-hour regimen, the coating reduced corrosion-related mass loss by 21%; under the 2-hour regimen, it reduced mass loss by 46%. Thus, increasing the frequency of transitions between wet and dry conditions magnified the coating's apparent benefit even though the total exposure to each condition remained unchanged.
Based on the text, which choice, if true, most coherently and directly accounts for the observation presented in the underlined sentence?
For an evidence question explaining an experimental result, first identify what the researchers held constant and what they changed. Then select the option that links the changed factor to the observed difference between groups. Reject choices that affect both groups proportionally, predict the opposite result, or depend on a feature favored by the other experimental regimen.
Hints
Focus on the experimental control
Note which quantities were held constant across the two regimens.
Consider the changing factor
Look for evidence involving how often wetting or drying begins, not merely how long either condition lasts.
Compare the sample types
The strongest explanation should show why repeated transitions increase damage more for uncoated steel than for coated steel.
Step-by-step Explanation
Identify what changed
The total immersion and drying times were controlled. Only the frequency of transitions changed, so an adequate explanation must depend on the number of transitions rather than simply on elapsed time.
Determine the required mechanism
The coating's benefit grew under the more frequent cycle. The best evidence must therefore describe a repeated source of damage that occurs mainly on uncoated steel each time a new drying period begins.
Select the evidence
The statement that directly supplies this mechanism is Choice C: During each drying period, evaporation briefly concentrates saltwater remaining on uncoated steel, while the coating prevents most saltwater from reaching the steel surface.