In the question below, a passage is given followed by a statement. You have to identify which assumption the argument in the passage is based upon. Passage: A study conducted in several cities found that traffic congestion is reduced significantly when a city implements a tolling system on its busiest roads. The study concluded that cities with high traffic congestion should consider implementing tolling systems to improve traffic flow and reduce travel time. Which of the following is an assumption on which the conclusion of the study is based?

The argument in the passage is that implementing tolling systems can reduce traffic congestion and thus improve travel times. This argument assumes that traffic congestion is the primary cause of long travel times. If this assumption is false, then reducing congestion through tolling may not necessarily lead to improved travel times. Therefore, E is the correct answer. A is irrelevant to the argument, as it talks about avoidance of toll roads, which is not considered in the study's findings. B introduces a new factor (air quality) that is not discussed in the study and is unrelated to the main conclusion about traffic flow. C is not necessary, as the conclusion is drawn based on the observed effect in the studied cities, not on their pre-tolling congestion levels. D is not directly necessary for the conclusion about tolling systems reducing congestion and travel time. Thus, the correct answer is E.