To those who said that Duolingo is saying this should be plural, that is incorrect. It is definitely singular (glitch or oversight, perhaps?).
Duolingo said I was wrong because I said "school or house" instead of "A school or a house." Basically, I left out the article "a." I disagree with this though because the words are given as a phrase, not a sentence, so there's no real indicator that an article was needed. If it was given as a sentence, then of course I'd put in the articles.
This wouldn't bother me as much though if Duolingo was at least consistent with article use--sometimes leaving out the "a" will still grant you a correct answer, and sometimes it won't. It seems arbitrary.