CREATE TABLE table_73474 ("No. in series" CLOB, "No. in season" CLOB, "Title" CLOB, "Directed by" CLOB, "Written by" CLOB, "Original air date" CLOB, "Production code" CLOB, "U.S. viewers  ( millions ) " CLOB)

Name the production code for number in season being 21
SELECT "Production code" FROM table_73474 WHERE "No. in season" = '21'
SELECT "Production code" FROM "table_73474" WHERE "No. in season" = '21'