LSAT 123 | Section 3 | Logical Reasoning: Q11

LR Question 11 Explanation

QUESTION TEXT: Feathers recently taken from seabirds stuffed and preserved…

QUESTION TYPE: Necessary Assumption

CONCLUSION: 100 years ago, saltwater fish had higher mercury levels.

REASONING:

• 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.

___________

1. 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.]
2. It doesn’t matter why fish have mercury. It only matters that they do.
3. This explains why the feathers have mercury. But it doesn’t explain the change in amount, so it’s irrelevant.
4. This would weaken the argument, by showing a potential difference between modern seabirds seabirds in 1880.
5. 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.