user: Question: What surface was the Città di Caltanissetta played on?
Columns: ['Outcome', 'Tournament', 'Surface', 'Partner', 'Opponents', 'Score']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Winner', 'Boca Raton , US', 'Hard', 'Andrey Kumantsov', 'Brian Battistone Dann Battistone', '7–5, 6–4']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Tournament" = 'Città di Caltanissetta';