Given a set of five words, generate the word from the set that does not belong (i.e. is the least relevant) with the other words. Words are separated by commas.

Q: streamline, sustainability, private, firm, stiff

A: stiff
****
Q: normal, commander, sargent, private, general

A: normal
****
Q: calm, positive, cool, tropical, popular

A:
tropical
****