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.
--------
Question: Category: U.S. RIVERS 
Clue: The Falls of Saint Anthony stop this mighty river from being navigable north of St. Paul, Minnesota

Answer: the mississippi


Question: Category: "D" IN SCIENCE 
Clue: Sharks first inhabited the waters during this period of the Paleozoic era named for an English county

Answer: devonian


Question: Category: U.S. HISTORY 
Clue: Formally organized in the 1870s, it began as a secret order of garment workers in 1869

Answer:
the knights of labor