What is the production code of the episode written by Cathryn Humphris?
CREATE TABLE table_24938621_3 (
    production_code VARCHAR,
    written_by VARCHAR
)
SELECT production_code FROM table_24938621_3 WHERE written_by = "Cathryn Humphris"
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: What is the production code of the episode written by Cathryn Humphris? ### Input: CREATE TABLE table_24938621_3 (
    production_code VARCHAR,
    written_by VARCHAR
) ### Response: SELECT production_code FROM table_24938621_3 WHERE written_by = "Cathryn Humphris"