How were expansion and migration related in the United States after 18

How were expansion and migration related in the United States after 1812?

a. As populations expanded, people migrated to rural areas outside of populated cities, creating the first suburbs.
b. As populations expanded, people migrated to and built up Eastern cities.
c. As the United States expanded its borders, people migrated to these new areas.
d. Expansion and migration were both direct results of improved water transportation after 1812.
e. Expansion and migration were both direct results of improved land transportation after 1812.

ANSWER
C