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.

[Q]: Category: elements 
Words: ground, replace, combustion, flame, fire
[A]: replace


[Q]: Category: direction 
Words: below, backward, melancholy, down, ground
[A]: melancholy


[Q]: Category: boats 
Words: head, galley, deck, check, lifeline
[A]:
check