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If a person chooses to walk rather than drive, there is one less vehicle emitting pollution into the air than there would be otherwise. Therefore if people would walk whenever it is feasible for them to do so, then pollution will be greatly reduced.

Which one of the following, if true, most strengthens the argument?


Correct answer: Nonmoving running vehicles, on average, emit half as much pollution per second as moving vehicles, but the greater congestion is, the more nonmoving running vehicles there are.


Why is this the correct answer? I don't see how it strengthens the argument at all.

It's a tough one. If a lot of the cars that are on the road are stuck in traffic, and non-moving cars still emit pollution, and the greater the number of cars on the road, the greater the chance you're stuck in traffic, then getting any cars of the road will lead to a lower chance of being stuck in traffic and also less pollution. This answer basically says that if you get cars off the road, not only will you reduce pollution by removing a pollution emitting vehicle from the road, but you're also lowering the chance that everyone else will be stuck in traffic, thereby reducing the pollution that the other cars emit.

Here's a fuller explanation.

Thanks!