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Question

Difficulty: EasyIdentifying Necessary Assumptions

City official: To reduce traffic congestion downtown, we should build a new subway line connecting the suburbs to the city center. A recent survey showed that 70 percent of suburban commuters would take the subway if it were available.

Which one of the following is an assumption on which the city official's argument depends?

  1. A
    Building the subway line will guarantee that traffic congestion downtown is entirely eliminated.
  2. B
    All of the suburban commuters who currently drive to the city center will switch to using the new subway line.
  3. At least some of the suburban commuters who would take the subway currently commute to downtown in vehicles that contribute to traffic congestion.Answer
  4. D
    The financial cost of constructing the new subway line is lower than the economic toll caused by the current traffic congestion.
  5. E
    Commuters who do not live in the suburbs will not use the new subway line to travel to the city center.

Answer

At least some of the suburban commuters who would take the subway currently commute to downtown in vehicles that contribute to traffic congestion.
The argument concludes that a new subway will reduce traffic congestion because 70 percent of suburban commuters will ride it. This reasoning contains a logical gap: it assumes that these future subway riders are currently using transportation methods that cause traffic. By applying the Negation Test, if we assume the opposite—that none of these commuters currently use vehicles that contribute to congestion (for example, they already carpool with others who will continue driving, or they take an existing train)—then moving them to a new subway would remove zero vehicles from the road, and congestion would not decrease. Because negating this statement destroys the argument's conclusion, it is a strictly necessary assumption.

Step-by-Step Solution

1
Identify the conclusion of the argument.
The conclusion is that building the subway will reduce traffic congestion downtown.
This establishes the logical target that the premises must support.
2
Identify the supporting premise.
The premise is that 70 percent of suburban commuters would take the subway if available.
Understanding the premise helps locate the logical gap between the evidence and the conclusion.
3
Find the logical gap between the premise and the conclusion.
The argument assumes that people switching to the subway will actually take cars or buses off the road. If they were already walking, biking, or taking a different train, congestion would not change.
Necessary assumptions bridge unstated gaps in the argument's logic.
4
Apply the Negation Test to verify the correct assumption.
Negating the correct statement means none of the new subway riders currently use vehicles that cause congestion. If true, the subway would remove zero vehicles from the road, destroying the conclusion.
A necessary assumption must be true for the argument to survive; its logical negation must completely undermine the argument.

Key Concept

Identifying Necessary Assumptions using the Negation Test
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Topics

Identifying Necessary Assumptions

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