What is the name of a partner that played on a hard surface on 18 April 2011?
CREATE TABLE "futures_titles" (
  "date" text,
  "tournament" text,
  "surface" text,
  "partner" text,
  "opponent_in_the_final" text,
  "score" text
);
SELECT "partner" FROM "futures_titles" WHERE "surface"='hard' AND "date"='18 april 2011';