CREATE TABLE table_21738 ("Series #" FLOAT, "Episode #" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "U.S. viewers  ( million ) " TEXT, "Original airdate" TEXT, "Production Code" TEXT)

What is the air date when the U.S. viewers was 5.50 million?
SELECT "Original airdate" FROM table_21738 WHERE "U.S. viewers (million)" = '5.50'
SELECT "Original airdate" FROM "table_21738" WHERE "U.S. viewers (million)" = '5.50'