The correct answer is - African Americans left the south in large numbers to work in war-related industries.
In this period a large portion of the African American population in search of a better life migrated from the south to the north of the country. And since it was a war period, big portion of the jobs available were in the war-related industries, so most of them started working into this facilities to make a living for themselves and their families. This internal migration in the United States caused big changes on regional and local level throughout the country from demographic point of view.