CREATE TABLE table_24938621_3 (production_code VARCHAR, written_by VARCHAR)

What is the production code of the episode written by Cathryn Humphris?
SELECT production_code FROM table_24938621_3 WHERE written_by = "Cathryn Humphris"
SELECT "production_code" FROM "table_24938621_3" WHERE "Cathryn Humphris" = "written_by"