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: SCIENCE GRAB BAG 
Clue: A material called URhGe can be manipulated into being this, lacking electrical resistance
superconductors

Category: 3-LETTER WORDS 
Clue: To udderly intimidate someone
cow

Category: DOCTOR 
Clue: In 1943 Dr. Willem Kolff was the first to use an artificial one of these organs on humans

kidneys