What was the airdate of 21 series number?
CREATE TABLE table_16965 (
    "Series no." real,
    "Title" text,
    "Directed by" text,
    "Written by" text,
    "Original air date" text,
    "Production Code" real
)
SELECT "Original air date" FROM table_16965 WHERE "Series no." = '21'
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 was the airdate of 21 series number? ### Input: CREATE TABLE table_16965 (
    "Series no." real,
    "Title" text,
    "Directed by" text,
    "Written by" text,
    "Original air date" text,
    "Production Code" real
) ### Response: SELECT "Original air date" FROM table_16965 WHERE "Series no." = '21'