who are the writers of the episode that had 3.07 millions of North American spectators?
CREATE TABLE "table1_21979779_1" (
  "no" real,
  "num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_million" text
);
SELECT "written_by" FROM "table1_21979779_1" WHERE "u_s_viewers_million"='3.07';