It's the habitual aspect, which is just hu- with no person/class prefix, but I forgot that it's not (officially) introduced until significantly lower in the tree. You do still see it pop up occasionally in some of the earlier exercises, though.
It should be an option for a correct answer because some of us are trying to skip certain activities, but we get counted wrong for things that are not incorrect.
Interesting. I guess I will see it at some point, but I'm already curious how it is used. Without a person or class prefix, is it just for third person passives, or maybe for a broader class of impersonal third persons?