PrepTest 123, Section 3, Question 11
Feathers recently taken from seabirds stuffed and preserved(62 more words not shown)
The argument depends on assuming(1 more words not shown)
- Athe proportion of(19 more words not shown)
- Bthe amount of(18 more words not shown)
- Cmercury derived from(11 more words not shown)
- Dthe stuffed seabirds(10 more words not shown)
- Ethe process used(18 more words not shown)
Reading the argument
How much mercury did the old feathers have before they were preserved?
That's what I'd want to know before using this comparison to conclude that fish contain more mercury now. The old feathers have half as much as the feathers from living birds. Maybe the fish really did contain less mercury a hundred years ago. But maybe the old feathers lost mercury during preservation. Then we'd be comparing an altered sample with a fresh one.
Put numbers on the problem. Suppose an old feather originally had 20 units of mercury, but the preservation process removed half. We now measure 10 units in it and 20 in a fresh feather. That looks like a doubling across time, even though both feathers started with 20. The difference could come from what happened to the old sample.
The passage gives us the fish as the source of the mercury in growing feathers. I can also ask whether those fish are saltwater fish, since that's where the conclusion goes. But the main issue I want to test is whether the old measurements still reflect the old levels.
What we’re looking for
The question asks for a required assumption. I'd expect something protecting that comparison: the old feathers weren't stripped of enough mercury to create the apparent increase. We need an assumption the reasoning relies on, not an extra claim about mercury that happens to sound scientific.
Working the answers
- A
The argument doesn't need the birds' diets to have shifted toward fish. It could claim fish contain more mercury even if the birds ate the same proportion of fish in both periods.
In fact, more fish in the modern diet could offer another explanation for more mercury in the feathers, without requiring more mercury in each fish. This is a possible change we'd want to investigate, not a change the author's conclusion depends on accepting.
- B
Where the mercury comes from isn't the conclusion. The argument could claim there is more mercury in fish now whether that change came from pollution or something else. A specific link to ocean pollution isn't required to make the comparison.
- C
Mercury can accumulate in feathers without being essential to their growth. The argument uses feathers as evidence of how much mercury the birds take in. It doesn't require mercury to help the feathers grow normally.
- D
Why would the old birds have to be young? The inference doesn't rely on their not being fully grown. Age could be something to investigate in designing a comparison, but this particular statement about the preserved birds isn't a commitment the argument needs.
- ECorrect
The comparison assumes preservation didn't substantially reduce the old feathers' mercury. If the process removed a substantial amount, their lower measured level could reflect that loss, rather than lower mercury in the fish they ate.
Our 20-to-10 example shows the problem: preservation alone can create the observed difference while the original levels stay equal. E blocks that explanation. It doesn't tell us everything we'd want to know about the birds' diets or the history of their environment; it protects a measurement the argument needs to rely on.
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