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_name_63 (rating VARCHAR, rank___number_ VARCHAR, air_date VARCHAR)
### Question:
What is the rating of the show ranked tba, aired on April 21, 2009?
### Answer:
SELECT rating FROM table_name_63 WHERE rank___number_ = "tba" AND air_date = "april 21, 2009"