PT38.S4.Q14 - reducing speed limits

http://7sage.com/lsat_explanations/lsat-38-section-4-question-14/
The conclusion is that reducing speed limits neither saves
lives nor protects the environment. The evidence is that the more slowly a car moves the
more time it spends on the road spewing exhaust and running the risk of collision.

My question is why is A wrong. The author assumes people follow the limits. If they ignore the limits then guess what, it doesn't matter if they reduce them. But more importantly, how is it we are suppose to see that the author is trying to assume that the more slowly a car is driven, the more time it spends on the road spewing [more] exhaust into the air.

You see the part in brackets is what you have to ASSUME the author to be saying, otherwise the question makes no sense. You read it and think it is a perfect argument. I don't understand how we are suppose to infer this. Otherwise it just looks like he is saying, well yeah, you drive slower, you spend more time on the road spewing exhaust but there is actually no indicator that the author thinks the exhaust expelled from the vehicle will be larger or more just from driving slower. Is this some kind of grammar trick?

Had it said drive slower, spend more time on the road, and shoot out more exhaust, then D would be perfectly logical. But that last part isn't there, so how do we infer it?