An explanation question about a study tests whether you can connect a possible cause to all the results. Read the observation and separate each result, especially when measurements come from different places or times. Then check what stayed the same between the groups, because a cause shared equally by both can't explain a difference on its own. Test whether each proposed cause can explain every result; sharing the study’s topic isn't enough.
Hints
- Hint 1
Read the underlined sentence as two results to explain. How does the swift stream’s concentration compare with the slow stream’s at each station?
- Hint 2
Check what the researchers kept the same, including the DNA released and the assay’s recovery rate. Which difference between the streams is left to consider?
- Hint 3
Trace each proposed cause from 20 meters to 600 meters. Ask whether it can explain the change in which stream has the higher concentration.
Step-by-step
Explain both readings
Step 1Identify the reversal
DNA concentration is 60 percent lower in the swift stream than in the slow stream at 20 meters, then 25 percent higher in the swift stream at 600 meters. What must a cause explain? The stream with the higher concentration changes, so the explanation must account for both readings.
- Step 2
Find what differs
Both streams receive identical quantities of synthetic DNA fragments per minute, and their dimensions, temperature, chemistry, and microbial communities match. What's different? The water moved four times faster in one stream than in the other. We're explaining what faster flow changes, not a difference in how much DNA the researchers released.
- Step 3
Test the distant result
What happens when we test the proposed causes at the distant station? With equal attachment rates based on distance, the same share would be lost along the same route, so which stream has the higher concentration wouldn't change. Turbulence that breaks fragments into undetectable pieces adds a source of DNA loss in swift water, which works against its higher concentration at 600 meters. And easier to detect as they age conflicts with the assay check: its recovery rate was verified to be equal for all samples.
- Step 4
Connect dilution and decay
The phrase greater discharge means more water passes each minute, so the same amount of DNA released produces a lower concentration nearby. Why does the swift stream pull ahead at 600 meters? Fragments reach that station sooner, leaving less time for decay and keeping more detectable DNA. Less decay can outweigh the initial dilution, making the distant concentration higher even though the nearby concentration was lower. Choice D.