What is the average episode that has September 11, 2007 as a region 1?
CREATE TABLE "dvd_releases" (
  "volume" text,
  "discs" real,
  "episodes" real,
  "region_1_release" text,
  "region_2_release" text,
  "region_4_release" text
);
SELECT AVG("episodes") FROM "dvd_releases" WHERE "region_1_release"='september 11, 2007';