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: secretary duty 
Words: correspondence, schedule, answer calls, keys, file
keys

Category: type of sandwich 
Words: submarine, burger, bar, lettuce, club
bar

Category: emotional status 
Words: sad, melancholy, backward, down, emotional upsets

backward