Self-study
Nuclear reactors are sometimes built in “geologically quiet” regions, so called by geologists because such regions are distant from plate boundaries and contain only minor faults. █████ ██ █████ █████ ██ █ ████████████ █████ ██████ ████████ ██ ██████████ ████ █████ ████ ████ ██ ███ █████ ████████████ ███████ ██ ███████ ████ ██ ███ █████████ ███████ ███████ █████ ██ ████ █ ███████ █████ ████ ███ █████ ██████ ██ ██ ██████ ██ ██ ██████████ ███ ████ ███████ ████ █ █████ ████ ███ ████████ ██ ██████████ ██████ ██████ ███████
Argument Breakdown
The stimulus is about building nuclear reactors in "geologically quiet" regions with only minor geological faults. This defines the domain of the argument.
Within this domain, the argument concludes that the sites with the lowest earthquake risk are those near a minor fault that has recently produced an earthquake. Why? Because the maximum earthquake frequency for these faults is one in 100,000 years. The idea is that since these faults have recently been active, they won't produce another earthquake for at least 100,000 years.
Objective: Identify a Necessary Assumption
There's an assumption we can spot in this argument related to the location of geological faults. The argument requires assuming that there are no sites without any faults at all. If there were, then presumably they would lower the earthquake risk to zero, which is even lower than one in 100,000 years.
We can hunt through the answer choices for this assumption, but it's always possible the correct answer will be more difficult to identify than we think. In that case, we can turn to process of elimination, using the negation and must-be-true tests to eliminate incorrect answer choices.
21.
Which one of the following ██ ██ ██████████ ██ █████ ███ ████████ ████████
a
Geologically quiet regions ███ ███ █████ █████████ ███████ ██ █████ ██ █████ ███████ █████████
There are two issues with (A). First, the argument isn't about overall danger, just about earthquake risk. An assumption about danger goes past what's necessary.
Second, geologically quiet regions are already locked in as the
b
For any potential ███████ ███████ █████ ███ ██████████ ██ █████ ██████ ██ ██ ██████████ ██ ███ ███████ ███████████ ██ ████ ███████
Much like (A), (B) goes past what's necessary by taking about "safety". The argument is
c
In a geologically █████ ███████ █████ █████████ ███████ ███████ ████ ██ ████ ██ █████ ███ █████ ██████
In other words, it's impossible to build a reactor that's not near any faults at all. This is the only way that building near a recently-active fault could possibly offer the lowest earthquake risk. Otherwise, if we could just build away from any faults, the argument wouldn't make sense.
d
Nuclear reactors that ███ ███████ ██ ████████████ █████ ███████ ███ █████ ██ █████████ ██ █████ ███ ███ ███ ███████████ ████ ████ ███ ██████████ ██ █████ ██ ████████ ██████
The argument is talking about the risk of an earthquake occurring, not about the likely outcome if one were to occur. Since the outcome of an earthquake is never discussed, it's not necessary to assume anything about how a reactor would handle such an event.
e
Earthquake faults in ████████████ █████ ███████ ███████ ███████████ ██ █████ ████ ██ ███████ ██████
In combination with the
We can see this using our handy negation test. To negate (E), we would say that at least some such faults could produce earthquakes less frequently than once in 100,000 years. For example, a certain fault could produce an earthquake only once in a million years. And yet, the argument would still make sense in saying that we minimize earthquake risk by building near recently-active faults, which are still guaranteed to have no earthquakes for at least 100,000 years.