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.

[EX Q]: Category: A WALK ACROSS THE CHARLES BRIDGE 
Clue: Originally called Stone Bridge, the Charles Bridge was built with this "stone", like the walls of the White House
[EX A]: sandstone

[EX Q]: Category: O FOR A 
Clue: Completely humble yourself for small stones & pebbles
[EX A]: grovel for gravel

[EX Q]: Category: THE PENDULUM 
Clue: This device uses an adjustable pendulum to indicate a given musical tempo either visually or aurally
[EX A]:
metronome