When there are  3.39 million u.s viewers what is the production code?
CREATE TABLE "table1_19068566_1" (
  "no" real,
  "num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "u_s_air_date" text,
  "production_code" text,
  "u_s_viewers_million" text
);
SELECT "production_code" FROM "table1_19068566_1" WHERE "u_s_viewers_million"='3.39';