"Why are you under the desk?"

[Glossmad]

I wonder why I was marked wrong for: γιατί είσαι εσύ κάτω άπο το γραφείο

[Moritz7698]

I had to pick from 3 greek options of which two were exactly the same except for one using "είσαι" and the other "είσαστε". I admit it is unlikely to have multiple people under the desk, but why is the second option wrong?