Possibly because it precedes a word starting with an "a" sound? Can a native speaker confirm?
Sorry, I only know 'no one works' and I know 'no work'. It would be helpful to describe your 'none work' a bit. Then German natives could get out what you want to say by 'none work'.
is this shorting used in formal English? I as a non-english-natives would say no(, you will know it better than me).
Niemand von uns(=of us) arbeitet./ Keiner von uns arbeitet.
It would be the same word in German as you already see in the Duolingo solution.
It's used pretty frequently in English, not necessarily only in formal situations. For example:
Person 1: How many people from the school studied at the library? Person 2: None.
Person 1: I can't believe the house burned down! How many people survived?
Person 2: None survived.
The phrasing is a little awkward in certain situations, but it's pretty frequently used as a euphemism (instead of saying "none of the people survived", saying "none survived" takes out the human element).
Which, I guess is my main question. Does "keiner" necessarily mean people, when used in the above example? I thought it might also apply to other creatures, as in "none of the animals work", which would then be short-handed to "none work", as an answer to a question.
Kein and all forms of 'kein' are the negation of 'a'/'ein'. Niemand is used for the number of zero people.