human
Considering the provided database schema and associated query, produce SQL code to retrieve the answer to the query. 
###Database Schema: Episode, Air Date, Timeslot, Rating, Share, 18–49 (Rating/Share), Viewers (m), Rank (#)
###Question: What is the rating of the show ranked tba, aired on April 21, 2009?
gpt
SELECT Rating FROM table WHERE Rank (#) = tba AND Air Date = april 21, 2009