单选题 8.Which of the following most logically completes the argument?
When officials in Tannersburg released their plan to widen the city's main roads, environmentalists protested that widened roads would attract more traffic and lead to increased air pollution. In response, city officials pointed out that today's pollution-control devices are at their most effective in vehicles traveling at higher speeds and that widening roads would increase the average speed of traffic. However, this effect can hardly be expected to offset the effect pointed out by environmentalists, since__________.

A、 increases in traffic volume generally produce decreases in the average speed of traffic unless roads are widened
B、 several of the roads that are slated for widening will have to be closed temporarily while construction is underway
C、 most of the air pollution generated by urban traffic comes from vehicles that do not have functioning pollution-control devices
D、 the newly widened roads will not have increased traffic volume if the roads that must be used to reach them are inadequate
E、 a vehicle traveling on a route that goes through Tannersburg will spend less time on Tannersburg's roads once the roads are widened

【正确答案】 C

【答案解析】Argument Evaluation
Situation Environmentalists protested a plan to widen a city's main roads on the grounds that it would increase traffic and air pollution. City officials replied that widening the roads would increase average traffic speeds, which would improve the effectiveness of vehicles'pollution-control devices.
Reasoning What would most support the conclusion that the improved effectiveness of the pollution-control devices would be insufficient to prevent the increased traffic from increasing air pollution? The word since preceding the blank space at the end of the argument indicates that the space should be filled with a premise supporting the conclusion stated immediately before the since. To support this conclusion, we would need evidence that widening the roads and increasing traffic speeds would not improve the pollution-control devices' effectiveness enough to compensate for the amount of added air pollution generated by the additional traffic on the widened roads.
A It is unclear whether traffic volume would increase if the roads were not widened. But if it did, this would cast doubt on the conclusion by suggesting that a combination of higher traffic volume and lower speeds could make air pollution worse if the roads were not widened than if they were widened.
B The argument is about the long-term effects of widening the roads, not about the temporary effects of closing them during construction.
C Correct. If most vehicles in the area lack air-pollution devices altogether or have ones that do not work, then it is highly questionable whether the greater efficiency of the few functioning devices would be sufficient to compensate for the increase in air pollution that would result from increased traffic.
D If anything, this casts doubt on the conclusion by suggesting that widening the roads may not increase traffic volume or air pollution at all.
E If anything, this casts doubt on the conclusion by suggesting that widening the roads will decrease the amount of time each vehicle spends generating air pollution on those roads.
The correct answer is C.