CREATE TABLE table_16781 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "U.S. viewers  ( millions ) " TEXT)

How many millions of U.S. viewers whatched episodes written by Krystal Houghton?
SELECT "U.S. viewers (millions)" FROM table_16781 WHERE "Written by" = 'Krystal Houghton'
SELECT "U.S. viewers (millions)" FROM "table_16781" WHERE "Written by" = 'Krystal Houghton'