I was curious about this too. What about the context tells me this is "you" and not "they"? I mean I would assume that THEY would not eat the other apple; apple being singular, but it's not nonsensical like some of the other questions that have been provided here.
Given that there is no real context provided I have noticed that the site will allow either translation, very nice considering it can be either one. Also note that if Sie is capitalized in the middle of a sentence it is automatically a formal you rather than they.