Given a category and a set of five words, find the word from the set that does not belong (i.e. is the least relevant) with the other words in the category. Words are separated by commas.
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Question: Category: type of rigidity 
Words: unbending, unyielding, inflexible, firm, streamline

Answer: streamline


Question: Category: elements 
Words: ice, fire, burn, let go, combustion

Answer: let go


Question: Category: animals 
Words: lion, duck, ostrich, push, geese

Answer:
push