What Did W

What did W.. E. B. Du Bois mean when he said that "the problem of the Twentieth Century is the problem of the color line"?

A) The long-standing racist thinking and behavior of white Americans would take hard work to overcome.
B) With the passing of white people who had living memories of slavery, racism would fade away.
C) If enough African Americans lived lives that white people would call "successful," then white people would eventually accept them as equal.
D) As Americans became more economically interdependent, especially as more people moved from rural to urban areas and left farming behind, they would prioritize economic solidarity over racial exclusivity.