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

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'