user: Question: On what surface was the tournament with opponents Kathleen Horvath Marcella Mesker in the finals?
Columns: ['Outcome', 'Date', 'Tournament', 'Surface', 'Partner', 'Opponents in the final', 'Score in the final']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Runners-up', 'April 29, 1985', 'Houston , USA WTA Virginia Slims', 'Clay', 'Helena Suková', 'Elise Burgin Martina Navratilova', '1–6, 6–3, 3–6']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Opponents in the final" = 'Kathleen Horvath Marcella Mesker';