Answer:
C. The increased importance of industrialization.
Step-by-step explanation:
The rapid development of cities was due to the growth of industrialization. Industrialization was increasingly demanding labor, so people moved from the countryside to the city in search of better living conditions and income. Particularly in the US, nineteenth-century industrial development ,and consequent urbanization ,were very much driven by the development of railways, which gave industry conditions to lower production and transportation costs in order to develop more rapidly.