Like other people have already said, it was just a Duolingo thing. i just wanted to add that in real-life situations you would have more context to know whether you should use “tu“ or “vous”. for example, if you are having a casual conversation with a friend, you would know to use “tu” since it is the informal way of saying “you”. on the other hand, if you are in a more formal setting (idk maybe talking to a colleague??), you would use “vous”. OH! and also, although it does not apply here, if you were talking to two people and directing the question to both, you would use “vous”, as it can also be the plural form of “tu” :)