What was the average rank for the film directed by roland emmerich under the studio of 20th century fox?
CREATE TABLE "highest_grossing_films_of_1996" (
  "rank" real,
  "title" text,
  "studio" text,
  "director" text,
  "worldwide_gross" text
);
SELECT AVG("rank") FROM "highest_grossing_films_of_1996" WHERE "studio"='20th century fox' AND "director"='roland emmerich';