Situation: The mayor of a town has a plan to reduce traffic: a five-dollar toll to enter the city. He believes this will reduce traffic congestion in the city; he bases this on the fact that bus fare to and from the city is far cheaper than the toll.
Reasoning: This is a weakener question--we are looking for a factor that will cause the plan to fail. Since the plan is to deter people from driving and get them to switch to the bus, we can attack from one of two angles: by showing that the bus is very unattractive to commuters, or by showing that the five-dollar fee isn't a very large burden. Either of those, if true, would make the mayor's plan unlikely to reduce traffic.
Choice (A) indicates that gasoline price increases will make riding into the city more expensive. That might make the mayor's plan unnecessary--but we're asking to make it fail, not to make it redundant. This is probably not the right answer. Choice (B), however, matches our prediction perfectly. If parking is already very expensive, adding a few dollars on top of that is unlikely to have much impact. Choice (C) tells us about current bus riders, not current drivers, so it's out of scope. Choice (D) actually strengthens the mayor's point, since it suggests that the $5 fee is a hardship on commuters. Finally, choice (E) offers no context with which to judge whether 20% is good or bad for the mayor's plan.
(B) is a good match for our prediction; all the rest are distortions or out of scope.
B is the correct answer.