What is federalism and can you give me an example of how it works?

How did the technology of the late 1800s affect the growth of cities? select one: a. cities were able to expand outward because of additions to railroads. b. electric streetcars enabled the wealthy city dwellers to move to the first suburbs. c. advances in steel and metal technology allowed for the development of the skyscraper, which allowed cities to grow upwards. d. all of these are correct.