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

The westinghouse air brake, first demonstrated successfully in 1868, had been developed to such an extent by 1887 that a train of 50 freight cars traveling at 20 miles per hour could be brought to a standstill in the space of 171 feet, whereas before they required 5 hand breakers over a distance of 1,500 feet for such a train to be stopped.

(a) whereas before they required5 hand breakers over a distance of 1,500 feet for such a train to be stopped.

ambiguous - are they standing 1500 feet apart or operating
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