What is the problem with A?
It has the right idiom, and as far as I can tell it's just a little wordy.
Is that it?
A is wrong because
The syntax is: X is twice Y <= X and Y must be parallel, if X is noun phrase, Y should be noun phrase too. Let see what X is. X = total amount of pollutant emitted annually by vehicles. This is noun phrase. But Y = that which is being emitted....... This is a clause (has both subject and verb). The parallel structure is not maintained.
"that which" is considered unidiomatic in GMAT.
Hope it helps.
Please +1 KUDO if my post helps. Thank you.
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