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: DUEL 
Clue: In English-speaking countries, once a duel was accepted, this person chose the weapons to be used

SOLUTION: the person being challenged

PROBLEM: Category: ASTRONOMY 
Clue: A galactic cloud of dust & gas, its name comes from the Latin for "cloud"

SOLUTION: nebula

PROBLEM: Category: QUESTIONS 
Clue: Robert Kennedy liked to say that some men see things & ask why, but he dreamed things & asked this

SOLUTION:
"why not?"