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 simplicity, answers should be in all lower cased letters.

Category: NORSE 
Clue: Skoll & Fenrir, both this type of fierce canine, will cause mayhem at Ragnarok, the end of the cosmos
wolves

Category: DRAMA 
Clue: Sula & Helena are robotesses in this Karel Capek play
"r.u.r."

Category: MUSICAL INSTRUMENTS 
Clue: You insert a roll into this instrument for it to tickle its own ivories

player piano