Answer :
It allowed for D- Greater mobility within big cities across the country and also allowed for suburbs to sprawl up and make travel within a city more doable
In the late 19th century, cities had a considerable growing population and mass transportation like the streetcar or the electric trolley were used as a solution to improve the mobility within big cities. Thus, the right answer is option D, greater mobility within big cities. Later on, cars became affordable, and they replaced the mass transportation vehicles, but nowadays many cities try to focus on electric mass transportation because it pollutes less than cars.