I got it wrong twice

Kaitlin got a train ticket valid for three cities: Athens, Rome, and Madrid. She can choose to visit some, none, or all of these cities. In the braces below, list all the possible sets of cities that Kaitlin can visit. Write each set in your list in roster form. If there is more than one set in your list, separate them with commas. If you need the empty set in your list, use the symbol . Athens, Rome, Madrid Athens Rome 0.0.... X G