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.

[EX Q]: Category: PARTICLE PHYSICS 
Clue: To get an element's mass number, add its protons & these
[EX A]: neutrons

[EX Q]: Category: HOW GREAT THOU ART 
Clue: This "mama's boy" answered the art critics in his 1890 book "The Gentle Art of Making Enemies"
[EX A]: whistler

[EX Q]: Category: ANIMAL WORDS & PHRASES 
Clue: Something very, very rare is "as scarce as" these chicken parts
[EX A]:
hen\'s teeth