A city argues that expanding public transit reduces congestion, so building new roads is unnecessary. The key assumption behind this argument is which of the following?

• A. Road construction reduces traffic as effectively as transit expansion.
• B. Congestion levels depend solely on road availability rather than other factors.
• C. Public transport has no impact on emissions.
• D. Road construction is always cheaper than transit improvements.

Answer: (B)

The main idea being tested is an implicit premise that congestion is determined solely by road availability. If congestion levels depend only on how much road capacity there is, then reducing demand through public transit would eliminate congestion and there’d be no need for new roads.

That’s why the correct choice is the one stating that congestion levels depend solely on road availability rather than other factors. If congestion were influenced by multiple factors beyond road capacity, expanding transit could help but wouldn’t guarantee that building new roads is unnecessary.

The other possibilities don’t fit the needed assumption. If road construction and transit had equal effects, the argument would rest on a comparison of effectiveness, not on what actually governs congestion. If public transport had no impact on emissions, that’s irrelevant to whether congestion is driven only by road capacity. If road construction were always cheaper, the conclusion wouldn’t hinge on what determines congestion, only on cost.