Answer:
B. Industrialization and urbanization
Explanation:
The nineteenth century is the period of time when the industrial revolution had taken a hold of society completely. Never in the history of the human civilization had there been such cooperation that significantly caused the progress of our species.
Industrialization caused a large number of people to settle in cities. Urbanization caused a large mass of people to live together. The dynamics of human interaction had changed.
In order to understand how social interactions were affected people started studying human behavior in the nineteenth century.
