What is the production code for the episode that had 18.07 million viewers? 
CREATE TABLE "table1_26198709_1" (
  "no" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "u_s_viewers_million" text,
  "rank_week" real,
  "original_air_date" text,
  "production_code" text
);
SELECT "production_code" FROM "table1_26198709_1" WHERE "u_s_viewers_million"='18.07';