CREATE TABLE table_24833 ("Series #" FLOAT, "Season #" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" TEXT, "U.S. viewers  ( millions ) " TEXT)

What is the production code for the episode with 8.56 million U.S. viewers?
SELECT "Production code" FROM table_24833 WHERE "U.S. viewers (millions)" = '8.56'
SELECT "Production code" FROM "table_24833" WHERE "U.S. viewers (millions)" = '8.56'