GMAT Sentence Correction (SC) | Re: The westinghouse air brake, first demonstrated successfully in 1868, h

Himikemcgarry ,

Can you please check my reasoning given below. In addition to your comments, is it safe to consider a 3-2 split in the answer choice based on the usage of 'before' and 'previously'. Once the splits are identified the answer can easily be narrowed down to B because of pronoun error in D.

(a) whereas before they required 5 hand breakers over a distance of 1,500 feet for such a train to be stopped. - Incorrect - 'they' incorrectly refers to freight cars. Also 'before' does not correctly