The sentence above is the translation for your question. Prepositions are merely "connector" words and in Ukrainian "у" can mean both "in" and "at". "я у офіс" can mean "I am at the office", or "I am in the office." The prepositions у, в, and на all mean in or at but in Ukrainian are used depending on place, location, establishment etc.
I would google it.
Oh ok I see. I don't think they've added every correct english solution yet as the course is still in beta. I've encountered similar problems throughout the course. Just report it.
"Today is my second day in school" is still wrong as of 12 October 2017. I've reported it.