CREATE TABLE table_28421 ("No." FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "U.S. viewers  ( million ) " TEXT, "Rank  ( week ) " FLOAT, "Original air date" TEXT, "Production code" TEXT)

What is the production code for the episode that had 18.07 million viewers?
SELECT "Production code" FROM table_28421 WHERE "U.S. viewers (million)" = '18.07'
SELECT "Production code" FROM "table_28421" WHERE "U.S. viewers (million)" = '18.07'