CREATE TABLE table_53991 ("Title" CLOB, "Directed by" CLOB, "Written by" CLOB, "Original air date" CLOB, "Production code" CLOB)

Who wrote the episode that has the production code ad1c05?
SELECT "Written by" FROM table_53991 WHERE "Production code" = 'ad1c05'
SELECT "Written by" FROM "table_53991" WHERE "Production code" = 'ad1c05'