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: employment activity 
Words: replace, ice, fire, ban, hire
ice

Category: elements 
Words: pay, cool, combustion, air, fire
pay

Category: boats 
Words: head, deck, gunwale, pack, crew

pack