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: LIBRARIES 
Clue: Samuel Pepys' famous diary is housed at the Pepysian Library at Magdalene College at this university
cambridge

Category: HITS OF THE 1960s 
Clue: In 1965 the McCoys hit No. 1 with this song about a girl who lives in a very bad part of town

"hang on sloopy"