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: MARK ANTONYMS 
Clue: The traditional mark of someone who can't sign his name has this opposite in tic-tac-toe
o

Category: WHAT SHOULD I "DU"? 
Clue: A philosophical system that seeks to explain all phenomena in terms of 2 distinct principles
dualism

Category: 20th CENTURY WOMEN 
Clue: Her work with mountain gorillas inspired the 1988 movie "Gorillas in the Mist"

fossey