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.

Category: type of rigidity 
Words: unbending, unyielding, inflexible, firm, streamline
streamline

Category: geographical property 
Words: mountain, flat, hill, mansion, plain
mansion

Category: secretary duty 
Words: answer calls, file, order office supplies, answer, software

software