I would like to know the explanation too, but also would like to give it a try.
Here I'll try to identify & omit wrong choices, let's start from 'A'. 'A' is a wrong choice as it can't be same because of "as much as that which is being" it's Wordy (Brevity issue).
now it can't be 'B' as "
as much annually as is" is awkward.
In 'C' it says "as much compared to" which is wordy it could have been "as compared to" no need for much, so it can't be C. Also by inserting "compared to" we r changing the meaning of sentence.
It can' be 'E' as again "compared to the" is changing the original meaning of the sentence. And also "compared to" is used for the comparison between two
unlike things to stress the resemblance and 'amount of pollutant' is being used for both of the areas which are "like" things
Now coming to D, first of all it's shorter & succinct one and also "that" is much better that "as much as" because "as much as" is used when 2 things are of equal amount or weight or volume etc. whereas "twice that" means twice of the pollutant emitted annually by vehicles at O'Hare International Airport.
So the Answer is "D" (succinct one)
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Please! correct me if I am wrong. Any addition or advice would be highly appreciable. I just tried to explain as much as I could, btw I am learning grammar too.