Which Process Has Served to Integrate National Societies Throughout the World

Which process has served to integrate national societies throughout the world in the post-colonial era?

A) The migration of peoples to foreign countries and to urban areas within their own countries.
B) The beginning of separatist movements that advocate racial separation from the larger group.
C) The practice of housing separate ethnic groups in specific urban areas, or ghettos.
D) Allowing all citizens of a particular country to vote in national elections.