C. A reduction in the number of European Immigrants
Explanation:
Due to WWI, there was a severe shortage of labor since many former factory workers had enlisted in the army and not many European migrants were pouring into the country. With demand for war materials at an all time high, African Americans were compelled to migrate to the more indutrial north to become industrial workers, with higher wages than those of an agricultural worker in order to make up for the lost manpower.
