The trend towards densely populated urban centers begun in 1800's continued into the 1900's. Man's development of urban centers was a major step away from what seemed to be nature's way of living: on farms and sparsely placed homesteads. Industrial production required hundreds of thousands of workers and, especially in the second industrial revolution, scientists. The urban centers that emerged during this period, such as Paris, London, and Berlin, were quickly changing the ratios of population from rural to urban Berlin's population, for example, went from 66% rural in 1871 to almost 66% urban before the first World War (see "The Second Industrial Revolution").
Answer:
B. Tigris and Euphrates river valley
Explanation:
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