【单选题】In the early 1800's, over 80 percent of the United States labor force was engaged in agriculture. Sophisticated technology and machinery were virtually nonexistent. People who lived in the cities and ...
A.
The transition from an agricultural to an industrial economy.
B.
The inventions that transformed life in the nineth century.
C.
The problems associated with the earliest factories.
D.
The difficulty of farm life in the nineth century,