The answer is: Cities
Approximately, around 80% of united states population live in the cities. This happen because most of job opportunities that exist in united states (whether it's art, technology, or manufacturing) are located in the cities.
Because of this, even people who born in rural areas start to move to inner cities in order to find jobs and sustain their living.
I think it might be d but please let me know if it is right
I think it's around the 1850s
