Starting in the late 1980s, a large number of migrants moved from Mexico to work in a meat packing plant in Marshalltown, Iowa. Back in Mexico, they experienced deteriorating economic conditions, and the meatpacking plant in the United States was in need of migrant laborers. This pattern of migration can be best understood as:

A. migrant network.
B. social capital.
C. push-pull theory.
D. melting pot.