LSAT 17 | Section 2 | Logical Reasoning: Q21

LR Question 21 Explanation

QUESTION TEXT: Nuclear reactors are somtimes built in “geologically quiet”…

QUESTION TYPE: Necessary Assumption

CONCLUSION: In a geographically quiet region the safest sites for nuclear reactors (in terms of earthquakes) are near those places where a minor fault caused an earthquake within living memory.

REASONING: Minor faults only cause earthquakes at most once every 100,000 years. (therefore if we recently had a quake there won’t be another for about 100,000 years)

ANALYSIS: This looks like a good argument. Earthquakes are very rare in geographically quiet regions. And minor faults can produce at most 1 quake per 100,000 years. So if a minor fault just had a quake then it will be safe for at least 100,000 years.

But nowhere does the stimulus tell us that there are minor faults everywhere in a quiet region. Perhaps there are spots not near any faults at all.

The conclusion is specifically about the risk of an earthquake within a geographically quiet zone. The wrong answer choices often talk about overall safety.

___________

1. The conclusion is only saying that within a geographically quiet region the spot least likely to be hit by an earthquake will be where an earthquake occurred recently. It doesn’t matter if there is a safer spot in a region that isn’t geographically quiet.
2. The conclusion isn’t talking about overall safety. It’s specifically about the likelihood of an earthquake.
3. CORRECT. If there are reactor sites not near any minor faults then those sites might be even safer from earthquakes.
4. The stimulus doesn’t say whether a reactor will survive an earthquake. It talks about the likelihood of an earthquake occurring near a reactor site.
5. The argument wouldn’t be affected if the faults produced earthquakes less frequently. It would only be hurt if earthquakes were more frequent.

Recap: The question begins with “Nuclear reactors are somtimes built in “geologically quiet””. It is a Necessary Assumption question. Learn how to master LSAT Necessary questions on the LSAT Logical Reasoning question types page.