CREATE TABLE table_77849 ("Episode" CLOB, "Air Date" CLOB, "Timeslot" CLOB, "Rating" CLOB, "Share" CLOB, "18\\u201349  ( Rating/Share ) " CLOB, "Viewers  ( m ) " FLOAT, "Rank  ( # ) " CLOB)

What is the rating of the show ranked tba, aired on April 21, 2009?
SELECT "Rating" FROM table_77849 WHERE "Rank (#)" = 'tba' AND "Air Date" = 'april 21, 2009'
SELECT "Rating" FROM "table_77849" WHERE "Air Date" = 'april 21, 2009' AND "Rank (#)" = 'tba'