Living in the suburbs and living in the cities was made possible by O Mass transit networks.
O The improvement of urban roads.
O The creation of residential neighborhoods outside of downtown areas.
O An increase in cheap, available housing.

Answer- Mass Transit Networks

Being able to move tons of people cheaply allowed for cities to be possible. Cities themselves have tons of people so traveling independently would be extremely difficult with the traffic. The railroads and steamboats were the most cost efficient and fastest ways to travel because it allowed for tons of people to board and did not cost much.