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: employment activity 
Words: fire, hire, ground, pay, retire

Answer: ground


Question: Category: moral characteristic 
Words: mild, equitable, unprejudiced, trustworthy, fair

Answer: mild


Question: Category: corporate 
Words: tough, sustainability, streamline, firm, leverage

Answer:
tough