Answer: A. Increasingly crowded urban areas
Explanation: Think about it this way. If you heard about more products being made in a city that means what? More jobs! Many people moved to cities and then made the cities unsanitary and many diseases spread around such as hepatitis. Anyways the more jobs there was the more money people could make and the more people lived in big cities like New York.
Answer:
B.
I could wrong that because there many theory.
I choose B that because my feelings think B make any sense?
