What type of surface was used for the carthage tournament on august 14, 1994?
CREATE TABLE "singles" (
  "date" text,
  "tournament" text,
  "surface" text,
  "opponent_in_the_final" text,
  "score" text
);
SELECT "surface" FROM "singles" WHERE "tournament"='carthage' AND "date"='august 14, 1994';