anks17 wrote:GMAT Experts,

Not sure how to arrive at the correct answer for attached question. Comparison by "that" seems bit odd but that happens to be the correct answer.

As with any SC question, this one requires us to pay attention to the non-underlined part.
We'll eliminate options one by one and end up with the correct answer.

I am going to mark the keywords in red, so that you can identify it easily.

Providing initial evidence that airports are a larger source of pollution than they were once believed to be, environmentalists in Chicago report that the total amount of pollutant emitted annually by vehicles at the O'hare International airport is twice as much as that which is being emitted annually by all motor vehicles in the Chicago Metro area

A) as much as that which is being emitted annually by all
This one is too wordy and awkward by GMAT standards. Also, incorrect usage of being. "that which is being". Eliminate.

B) as much annually as is emitted by the
This one uses annually redundantly in the first part of the comparison and omits it in the second part. Eliminate.

C) as much compared to what is annually emitted by all
The correct expression is "twice as much as". Incorrect expression. Eliminate.


D) that emitted annually by all
This one is concise and correct. "That" here refers to "the total amount of pollutant".
Also,
"The total amount of pollutant emitted annually by" is parallel to
"That emitted annually by". This is the best answer. Choose this.

E) that emitted annually compared to
This one again is just purely awkward. (Also I don't remember seeing "That + verb + infinitive form" ever). Eliminate.

Cheers!

Thank you Eagleeye.