PrepTest 121, Section 1, Question 25
Almost all microbe species live together in dense,(48 more words not shown)
Which one of the following,(10 more words not shown)
- AIt is currently(23 more words not shown)
- BIf it is(33 more words not shown)
- CNo microbiologist can(17 more words not shown)
- DAt least some(15 more words not shown)
- ENo microbe species(18 more words not shown)
Reading the argument
Do we have to grow a species by itself to understand it completely?
We know why these microbes are hard to separate. They live in communities, support each other's environment, and regulate their populations through chemical signals. The passage tells us we currently can't cultivate one of these species in isolation. Accept that limitation. It doesn't yet tell us what scientists can know.
Maybe studying a microbe within its community could still teach us everything about it. I'm not claiming scientists can actually do that. I'm asking what rules it out. The author moves from an inability to grow the species alone to incomplete knowledge of the species without supplying the connection.
What we’re looking for
We're asked to make the conclusion follow. So add the requirement: complete knowledge of a species requires being able to cultivate it in isolation. That would turn the stated practical limitation into the limitation on knowledge that the conclusion claims.
Working the answers
- A
This offers another difficulty with reproducing the communities' chemical systems. It doesn't establish a limit on knowledge. Even accepting that scientists cannot reproduce those systems, we still need a reason that observing or studying them cannot yield complete understanding.
- B
This connects inability to reproduce the community's conditions with inability to cultivate a species alone. But we already have the latter fact. What remains missing is the connection from isolated cultivation to complete knowledge. Explaining a premise more fully doesn't automatically supply the step from that premise to the conclusion.
- CCorrect
Complete knowledge now requires the ability to cultivate the species in isolation. The microbes in these communities cannot currently be cultivated that way, so microbiologists cannot have complete knowledge of them under C's rule.
The passage says almost all microbe species live in such communities. That covers the “most” in the conclusion. C can state its rule broadly for any organism; we only need to apply it to the many microbe species described here. The possible exceptions outside these communities don't stop the argument from reaching its claim about most species.
- D
Some microbiologists lacking complete knowledge doesn't establish the limitation for microbiologists generally. Those researchers might know little while other researchers have complete knowledge. D also doesn't connect the cultivation problem with the asserted lack of knowledge. A weaker statement about some people's ignorance won't make the full conclusion follow.
- E
Inability to survive outside the community is another biological limitation. It doesn't tell us what can be learned about a species while it lives inside that community. We're trying to establish incomplete knowledge, so a restriction on where the species can survive still needs the connection C provides.
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