What is the name of the leader of the South African vocal group Ladysmith Black Mambazo, who co-wrote the Graceland songs “Diamonds on the Soles of Her Feet” and “Homeless” with Simon.

a) Youssou N’Dour
b) Ray Phiri
c) Joseph Shabalala (the other three are all African musicians who played on the album)
d) Bakithi Kumalo