Answer: D. in the poorest neighborhoods of major cities.
Step-by-step explanation:
Immigrants moved to the United States in large numbers during the 1800s and early 1900s in order to take advantage of the "American dream" that they had heard so much about.
When they reached the shores of America however, what they found was that you need to work really really hard if you came with nothing or little.
When it came to matters of accommodation, they found that the only places they could afford were in the poorest neighborhoods in the city and so they moved in and made these places their own.