CREATE TABLE table_74172 ("Series #" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" FLOAT, "U.S. viewers  ( million ) " TEXT)

What is the original air date when the u.s. viewers in millions was 5.39?
SELECT "Original air date" FROM table_74172 WHERE "U.S. viewers (million)" = '5.39'
SELECT "Original air date" FROM "table_74172" WHERE "U.S. viewers (million)" = '5.39'