Let us break the argument down -

Environmentalists -
Widened roads --> More traffic --> More air pollution.

City Officials -
Widened roads --> Higher speeds --> pollution-control devices more effective --> Less Air pollution.

We have been asked to weaken the city officials' claims.

A - Incorrect.
this option basically states that -
if roads are not widened, then increase in traffic volume leads to decrease in average speed.

this reinforces city officials' argument.

B - Incorrect.
even though this action might lead to increased traffic volume and thus reduced average speeds on other roads, the effect is only temporary. If other roads can handle increased traffic, then there might not be a noticeable increase in pollution.

C - correct answer.
this implies that even if the average speeds on such roads increase, pollution may not decrease. Because most of the vehicles do not use pollution-control devices.
Increase in traffic will only make the situation worse.

D - incorrect.
We do not know whether the roads used to reach the newly widened roads are inadequate or not.
If they are inadequate, then there won't be any increase in traffic volume. this kind of strengthens the city officials' argument.

E - incorrect.
"spend less time" implies that average speeds on those roads will be higher. Hence, less air pollution. this again seems to strengthen the city officials' argument.

NandishSS wrote: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

City officials' premise:
Today's pollution-control devices are most effective at higher speeds, and widening the city's roads will increase the average speed of traffic.
City officials' conclusion:
Widening the city's roads will not lead to an increase in air pollution.

The correct answer choice must WEAKEN the city officials' conclusion.

C: most of the air pollution generated by urban traffic comes from vehicles that do not have functioning pollution-control devices
Here, urban vehicles lack functioning pollution-control devices, WEAKENING the city officials' conclusion that these devices will prevent an increase in the city's air pollution.

The correct answer is C.