You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For consistency, answers should be in all lower cased letters.

Category: FILTHY RICH 
Clue: Worth over $15 billion, Robson, son of Sam, runs this company, the USA's largest private employer
wal-mart

Category: A NOSE FOR GNUS 
Clue: A main predator of the adult gnu, it has a mane, too
lion

Category: SIXTEENTH 
Clue: In 217 A.D. Callistus I became the sixteenth man to hold this lofty post

pope