QUESTION TYPE: Necessary Assumption
CONCLUSION: 100 years ago, saltwater fish had higher mercury levels.
- Mercury in bird feathers comes only from the saltwater fish those birds eat.
- Birds stuffed and preserved in the 1880s have only half the mercury in their feathers that modern birds do.
ANALYSIS: This argument ignores a few possibilities:
- Maybe mercury degrades over time
- Maybe the preservation method gets rid of mercury
- Maybe the birds ate less fish in the 1880s
It is necessary to assume that all of these are false. Note that A is a trap: it restates my third possibility. The opposite of A would be a necessary assumption.
- If the birds ate things other than fish, that would offer a different explanation for why mercury levels are lower. This weakens the argument.
Negation: Seabirds ate at least as much fish in the 1880s as now.
[This negation is actually a necessary assumption. It should be the answer that is, not the negation.]
- It doesn’t matter why fish have mercury. It only matters that they do.
- This explains why the feathers have mercury. But it doesn’t explain the change in amount, so it’s irrelevant.
- This would weaken the argument, by showing a potential difference between modern seabirds seabirds in 1880.
- CORRECT. If you negate this, it wrecks the argument. The mercury level would be lower due to the process, not due to lower mercury levels in fish.
Negation: The preservation process substantially decreased the amount of mercury in the birds’ feathers.
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