PrepTest 154, Section 4, Question 26
Current evidence indicates that there is no methane(34 more words not shown)
The conclusion drawn above follows(9 more words not shown)
- AThere is, in(6 more words not shown)
- BIf methane exists(12 more words not shown)
- CIf there is(15 more words not shown)
- DIf there are(13 more words not shown)
- EIf there is(12 more words not shown)
Reading the argument
No microbes doesn't automatically mean no life. The evidence indicates that Planet 253 has no methane. The author says that, if this is right, the planet certainly has no life. Why would methane be required for life? The explanation that follows concerns microbes: they always produce methane, so their presence would give us methane. If there is none, there can't be microbes.
That gets us as far as no microbes. But could some other kind of life exist without microbes on this planet? We don't need to know whether that happens in the real universe. The argument has simply changed from a specific kind of life to life generally without providing the connection.
What we’re looking for
For a rule that makes the conclusion follow, link those two claims: if there are no microbes on Planet 253, there is no life there. Then we can follow the whole path. No methane rules out microbes; no microbes rules out life. The added rule needn't say microbes are the only kind of life. It just has to rule out life existing there without them.
Working the answers
- A
This confirms the no-methane condition, but the argument still needs to get from that fact to no life. We already have a reason no methane means no microbes. Making the methane evidence certain doesn't show that every form of life depends on microbes being present. The gap remains.
- B
Detectability concerns whether methane could escape our notice. That might matter when assessing the evidence of its absence, but the conclusion is conditional on there being no methane. Even granting that absence, we still need to connect no microbes with no life. B doesn't supply that connection.
- C
Whether methane existed in the past is a different question. We are trying to establish what follows about life now if methane is absent now. A planet with no past methane could still leave us asking whether life without microbes is possible. Extending the methane claim through history doesn't resolve the gap.
- DCorrect
This is the missing link. With no methane, there are no microbes, because microbes would produce methane. D then tells us that the lack of microbes means a lack of life on this planet. The conclusion now follows.
Keep the rule's meaning precise: other kinds of life could exist alongside microbes. What D excludes is life on Planet 253 when there are no microbes. That is enough for this argument.
- E
This tells us what follows if methane is present. Our situation is its absence. Even if methane guaranteed life, life might still exist in some other circumstances without methane. E doesn't establish that methane or microbes are required for life, so it can't support the conclusion we need.
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