Today, few rap music fans take the time to dig into the genre's history. They have grown up in a time in which rap seems strikingly different from other forms of popular music. Because of this, they fail to realize that the sound of contemporary rap is based on a wide variety of genres. Rap rests firmly on a substratum of blues, folk, rock and roll, and various experimental forms of music that developed during the middle of the twentieth century. As used in the paragraph above, what does the word substratum mean? A. foundation B. association C. fraction D. assembly