SELECT "Rating" FROM table_77849 WHERE "Rank (#)" = 'tba' AND "Air Date" = 'april 21, 2009'
What is the rating of the show ranked tba, aired on April 21, 2009?
CREATE TABLE table_77849 (
    "Episode" text,
    "Air Date" text,
    "Timeslot" text,
    "Rating" text,
    "Share" text,
    "18\u201349 (Rating/Share)" text,
    "Viewers (m)" real,
    "Rank (#)" text
)