What was the main reason behind the first wave of the Great Migration during World War I, prompting African Americans to leave the South?
A.) the rise of the Ku Klux Klan, which only existed in southern states
B.) the appeal of sharecropping, where they could work the land but not own it
C.) a booming increase in oil production and refinement in Oklahoma
D.) a need for factory workers in large cities to help produce machinery and supplies