Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_23177573_1 (original_air_date VARCHAR, rank__week_ VARCHAR)
### Question:
What is the original air date of the episode that had a week ranking of 21?
### Answer:
SELECT original_air_date FROM table_23177573_1 WHERE rank__week_ = 21