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: I WROTE THAT! 
Clue: "Ottsy i Deti", aka "Fathers and Sons"
ivan turgenev

Category: TWINS 
Clue: Brigitte Helm played both the virginal Maria & her sexy robot twin in this 1927 Fritz Lang film

metropolis