单选题根据下面的内容，回答题.

What does the passage mainly discuss?

A．The economic development of the Unites States in the eighteenth century.
B．Ways in Which economic development led to social changes in the United States.
C．Population growth in the western United States.
D．The increasing availability of industrial jobs for women in the Unites States.

A．emigrated to other countries.
B．often settled in the West.
C．tended to change the place in which they lived.
D．had a higher rate of birth than ever before.

A．A highly conservative society that was resistant to new ideas.
B．A society that was undergoing fundamental change.
C．A society that had been gradually changing since the early 1700's.
D．A nomadic society that was starting permanent settlements.