user: Question: What is the rating of the show ranked TBA, aired on April 21, 2009?
Columns: ['Episode', 'Air Date', 'Timeslot', 'Rating', 'Share', '18–49 (Rating/Share)', 'Viewers (m)', 'Rank (#)']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'real', 'text']
Sample row: ['" A New Hope "', 'March 3, 2009', 'Tuesday 8/7c', '8.5', '9', '0.9/2', 2.35, 'TBA']
SQL:

assistant: SELECT "Rating" FROM "Table" WHERE "Rank (#)" = 'TBA' AND "Air Date" = 'April 21, 2009';