CREATE TABLE table_29804 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" CLOB, "Directed by" CLOB, "Written by" CLOB, "Original air date" CLOB, "Production code" FLOAT, "U.S. viewers  ( millions ) " CLOB)

What is the production code for the episode that had 23.9 million u.s. viewers?
SELECT "Production code" FROM table_29804 WHERE "U.S. viewers (millions)" = '23.9'
SELECT "Production code" FROM "table_29804" WHERE "U.S. viewers (millions)" = '23.9'