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: corporate 
Words: corporation, firm, synergy, tough, concerted

A: tough
****
Q: Category: physical activity 
Words: trip, jump, fall, equinox, exercise

A: equinox
****
Q: Category: animal 
Words: ant, train, walk, fly, grasshopper

A:
train
****