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

Category: MUSIC 
Clue: Webster's defines this word as "a chord of three tones consisting of a root with its third and fifth"
a triad

Category: LET'S GET IN SHAPE 
Clue: President's Park South, just south of the White House, is also called this for its shape, a flattened hoop

the ellipse