user: Question: What surface was used on May 13, 2007?
Columns: ['Date', 'Tournament', 'Surface', 'Opponent in the final', 'Score']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['August 14, 1994', 'Carthage', 'Clay', 'Anne-Gaëlle Sidot', '5–7 6–3 6–4']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Date" = 'May 13, 2007';