The two largest groups of migrants to american cities during world war ii were
a) japanese americans and african americans.
b) mexican americans and southern whites.
c) african americans and mexican americans.
d) native americans and chinese americans.
e) southern whites and japanese americans.

I would say African Americans and Mexican Americans. 

Because In Fighting Their Own Battles, Brian D. Behnken explores the histories of the African American and Mexican American Civil Rights Movements in Texas from the post-World War II era to the early 1970s. He argues that concepts of race prevented attempts to build a united movement between the two communities. He also contends that cultural dissimilarities, class tensions, organization, and tactical difference and geographical distance reduced the potential for cooperative ventures between African Americans and Mexican Americans. Throughout his study Behnken shows how both communities worked simultaneously but separately to challenge racial discrimination and segregation in Texas.

The answer is crop yield.