Oh; I see. I typed it as masculine, and it was also marked "correct". :)
In general, though, if you are talking about a group of mixed-gender objects or people, or if it is unknown, you use the masculine.
That makes sense - I mean, it's the same as in Spanish. Maybe it was a bug in the answer key, or maybe I'm misremembering what I saw - but I don't think that's the case.
If I have an international keyboard on a Windows 7 computer, how would I get è? I know an apostrophe and an e gets é but how do I get it the other way?