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.
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Question: Category: WHAT SHOULD I "DU"? 
Clue: A philosophical system that seeks to explain all phenomena in terms of 2 distinct principles

Answer: dualism


Question: Category: WHAT THE "L"? 
Clue: When you sprain your ankle, you tear or twist these tissues that connect the leg & foot bones

Answer: ligaments


Question: Category: GEOLOGY 
Clue: It's a valley formed between 2 parallel faults; there's a "Great" one in the Eastern Hemisphere

Answer:
a rift valley