TASK DEFINITION: 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.
PROBLEM: Category: A DELICATE OPERATION 
Clue: A bypass operation may bypass the carotid or coronary one of these

SOLUTION: artery

PROBLEM: Category: PRESIDENTIAL NICKNAMES 
Clue: "Butcher from Galena"

SOLUTION: grant

PROBLEM: Category: "P.B." 
Clue: Anna Pavlova or Dame Margot Fonteyn, for example

SOLUTION:
prima ballerina

