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: ANIMALS
Clue: These mollusks, not oysters, are the most common source for freshwater pearls
mussels

Category: LITERARY TRILOGIES
Clue: "Number One" is actually the number two novel in a trilogy by this author of "U.S.A."
john dos passos

Category: MOUNTAINS
Clue: South America's Apurimac & Marinon Rivers rise in the permanent snows & glaciers of this range
the andes