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.

[Q]: Category: TV BEFORE & AFTER 
Clue: Jack Bauer saddles up & joins 3 friends bringing about the end of the world
[A]: 24 horsemen of the apocalypse


[Q]: Category: LITERATURE 
Clue: The first sequel to this classic French novel was titled "Twenty Years After"
[A]: the three musketeers


[Q]: Category: KNOW YOUR EARTH 
Clue: Giovanni Arduino classified rocks oldest to newest as primary, secondary & this, a term still used
[A]:
tertiary