What important contribution the Cecil Sharp make to our knowledge of American folksongs?

A$.$ Sharp documented and collected Child Ballads that were still being sung by working class people in Appalachia in the early $1900$ s$.$

B$.$ Sharp documented indigenous American folk songs that had developed apart from any other folksong traditions.

c$.$

Sharp was among the first folksong collectors to be interested in African American music.

D$.$ Sharp argued that American popular music was rooted in British ballads.