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: type of rigidity 
Words: tough, taut, rough, amalgamated, firm
[A]: amalgamated


[Q]: Category: weather 
Words: brisk, ghostly, windy, fair, deal
[A]: ghostly


[Q]: Category: mode of transportation 
Words: train, glide, fly, drive, dive
[A]:
dive