user: Question: The game played 31 May 2010 was played on what surface?
Columns: ['Outcome', 'Date', 'Tournament', 'Surface', 'Opponent', 'Score']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Runner-up', '12 November 2007', 'Helsinki , Finland', 'Hard (i)', 'Steve Darcis', '3–6, 6–1, 4–6']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Date" = '31 May 2010';