What was one reason why cities grew rapidly in the United States as a big business ?

A) Companies did not want to build factories in rural areas because farm workers often destroyed their property.
B) Large companies built company housing that they gave to workers in cities for free. That attracted many immigrants to the cities.
C) People moved into cites to escape the worsening rural violence between the army and Native Americans.
D) Larger companies could produce goods at a lower cost. As companies grew, they needed a large supply of workers. The immigrants and others moving into large cites supplied the labor the large companies needed.

I think its D.