Three could be honestly what you want and the answers are very vague.
Answer:
foosil fuels,and hydroelectricity energy
Explanation:
The main change is the countries the immigrants were coming from. The immigrants in 1840s were coming from the Western Europe mainly Ireland and Germany. Immigrants from Asia and Eastern Europe were coming a bit later while during the beginning of the 20th century most of the immigrants came from Italy.
Vaccination to prevent diseases. Vegan diets or whatever diets will not prevent them from having a better immune system to fight off such diseases
China, Hong Kong, Vietnam and many other small countries
