Why wouldn't "are" be as equally correct as "is", depending on specific context?
Both are accepted. However, Duolingo is doing something fishy here as the main translation in the system is different from that one shown here. I will try to force some update.
I just got this as a “word bank” exercise, and there was an “are” brick but no “is” brick. So I would assume that “are” is an accepted answer now.
It has always been accepted. Well, at least supposed to be.