What's the production code of the episode seen by 11.64 million US viewers?
CREATE TABLE "table1_12159115_2" (
  "series_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_millions" text
);
SELECT "production_code" FROM "table1_12159115_2" WHERE "u_s_viewers_millions"='11.64';