Answer: A. Cities grew rapidly as people moved from rural areas to work in industry.
Futher detail:
The Industrial Revolution had its beginning in Great Britain, and eventually spread from there. Once the United States became involved, the size and resources of the country allowed the US to become a bigger industrial power than the nations of Europe.
Industrialization also led to the phenomenon of urbanization -- the movement of people away from the rural countryside and into cities. That also led to other issues, like sanitation and crime problems in cities. So sanitation and health measures were enacted, and the first police forces were formed. For instance, in 1838, Boston established the first fully organized police force with full-time on-duty officers, paid as public servants. By the 1880s, all major US cities had established regular police forces.