For a question asking what accounts for an observation, divide the observation into all of its required parts and test each option against each part. When the relationship reverses across locations or conditions, look for an explanation involving competing effects; reject choices that predict the same direction throughout.
Hints
- Hint 1
Consider the entire pattern
Do not evaluate only the result at 20 meters. The explanation must also account for why the comparison reverses at 600 meters.
- Hint 2
Look for opposite effects
Identify the choice containing two mechanisms whose relative importance could change between a nearby station and a distant one.
Step-by-step
- Step 1
Identify the pattern
Near the release point, the swift stream has the lower DNA concentration. Farther downstream, the ranking reverses: the swift stream has the higher concentration.
- Step 2
Look for competing effects
A successful explanation must provide one effect that lowers the initial concentration in the swift stream and another effect that gives that stream an advantage over a long distance.
- Step 3
Match the evidence
The greater discharge dilutes the fragments near the source. However, the fragments reach the distant station more quickly in the swift stream, leaving less time for decay. These competing effects account for the observed reversal.