CREATE TABLE table_50811 ("Edition" TEXT, "Round" TEXT, "Date" TEXT, "Against" TEXT, "Surface" TEXT, "Opponent" TEXT, "Result" TEXT)

what is the surface when the opponent is selma babic?
SELECT "Surface" FROM table_50811 WHERE "Opponent" = 'selma babic'
SELECT "Surface" FROM "table_50811" WHERE "Opponent" = 'selma babic'