Tell me the surface for 16 april 2007
CREATE TABLE "titles_6" (
  "outcome" text,
  "date" text,
  "tournament" text,
  "surface" text,
  "partner" text,
  "opponents_in_the_final" text,
  "score_in_the_final" text
);
SELECT "surface" FROM "titles_6" WHERE "date"='16 april 2007';