CREATE TABLE table_51868 ("Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" TEXT)

tell me the writer of production code ad1d09.
SELECT "Written by" FROM table_51868 WHERE "Production code" = 'ad1d09'
SELECT "Written by" FROM "table_51868" WHERE "Production code" = 'ad1d09'