Remember that this course is also for people that don't speak English as their mother tongue. If the correct English answer is shown, even if you giive one that is not grammatically nice but acceptable, I would keep it. People get indeed frustrated when learning a language and scolded for a minor thing in the language they must use as a mean.
Yes you can say "sul", but as several people have pointed out, language is not tit for tat exchanges of words. Understanding the culture and history of different countries helps "get" the language. Italian is much less structured and formal than many, many other languages. Like German. And if you get the chance (EVER!) to go to Italy and practice speaking Italian, by all means GO. Italians are friendly, helpful (just don't ask for or rely on directions from them) , and they love showing off their country. I swear to God, if you get lost and have no place to stay, they will take you in or at the very least they have a friend whose uncle owns a little hotel....and if you go to see long lost relatives, they will not let you buy food, they take you everywhere, and show you everything....it's amazing!!