According to the author of "Hip-Hop Planet", which of the following increased as hip-hop music became more commerical?
A. The assassinations of African American leaders.
B. The riots in major cities, such as the one in Paris, which was a result of mistreatment of French settlements in Africa.
C. The sales of cassette tapes and CDs in retail outlets nationwide.
D. the popularity of the music among working class and suburban white teenagers in America.