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 consistency, answers should be in all lower cased letters.
Q: Category: DON'T LISTEN TO THEM!
Clue: In 1899 Charles Duell, the head of this U.S. "Office" said, "Everything that can be invented has been invented"
A:
u.s. patent office