A. in the 1920s-1930s due to an increase of population
Explanation:
In the first half of the twentieth century, both the United States and many European countries began to see more generalised expansion of suburbs beyond the inner city areas. For this reason, most large cities today have a first inner ring of suburbs that targeted middle and lower-middle class households who relied principally on public transportation to get them to their jobs in the central and downtown areas.
