PrepTest 102, Section 3, Question 4
Scientists analyzing air bubbles that had been trapped(67 more words not shown)
Which one of the following,(9 more words not shown)
- ADiatoms are a(14 more words not shown)
- BComputer models suggest(16 more words not shown)
- CThe dust found(17 more words not shown)
- DSediment from the(26 more words not shown)
- EAlgae that currently(22 more words not shown)
Reading the argument
If there was a great increase in algae, where is the evidence of all that algae?
The air bubbles give us two findings: unusually much ferrous material and unusually little carbon dioxide. I might initially wonder how well trapped air preserves the old atmosphere. Could its contents change over time? That's a question about the evidence, not a fact we're entitled to assume.
The scientist offers a particular explanation of the findings. Algae absorb carbon dioxide. Perhaps the ferrous material helped algae multiply, and the larger algae population removed more carbon dioxide from the atmosphere. I can follow that proposed chain, but we haven't yet established that it actually happened.
What else could explain the low carbon dioxide? Trees are another possible place to look for something absorbing it. We'd need evidence that such an alternative fit the time and setting; naming it alone wouldn't establish a rival explanation.
What we’re looking for
We're asked to weaken the scientist's hypothesis. I'd look either for a supported alternative cause or for evidence against the proposed algae increase. A record showing no increase where we'd expect to see one would give us a reason to question the story. Our target is the hypothesized growth in algae, not a denial that the air bubbles contain the reported materials.
Working the answers
- A
Diatoms remaining largely unchanged doesn't tell us how many existed during the ice age. Their form could stay the same while their population rose, fell, or stayed steady. We need evidence against the proposed population increase, not simply information about how little the organisms evolved.
- B
This makes the proposed link more plausible. If models indicate that extra ferrous material can promote algae growth today, that supports the idea that the material could help algae multiply. It doesn't prove what happened during the ice age, but it points toward the scientist's mechanism rather than against it.
- C
Other minerals being present doesn't by itself give us another explanation. Do those minerals affect algae or carbon dioxide? C never says. They could have been present while the ferrous material still produced the proposed increase. To turn their presence into a weakener, we'd have to invent an effect the answer doesn't supply.
- DCorrect
Diatoms leave shells when they die. If their population greatly increased, we'd expect that increase to leave some sign in the rate at which those shells accumulated. But the ocean-floor sediment near Antarctica shows no increase in that rate during the relevant period.
That is evidence against a consequence we'd expect from the scientist's account. It doesn't establish that there were no diatoms at all, and it doesn't prove that no other explanation of the shell record is possible. The important contrast is a predicted great population increase without the corresponding increase in remains. That makes the hypothesis less convincing.
- E
Not being harmed isn't the same as being helped to grow. Algae could tolerate extra ferrous material without multiplying in response to it. And this answer concerns present algae, not direct evidence of an ice-age population increase. It supplies no reason to think the scientist's hypothesized increase did not occur.
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