user: Question: What is the original air date of the episode that had a week ranking of 21?
Columns: ['No.', '#', 'Title', 'Directed by', 'Written by', 'U.S. viewers (million)', 'Rank (week)', 'Original air date']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'real', 'text']
Sample row: [113.0, 3.0, '" Epic Fail "', 'Greg Yaitanes', 'Sara Hess & Liz Friedman', '14.44', 1.0, 'September28,2009']
SQL:

assistant: SELECT "Original air date" FROM "Table" WHERE "Rank (week)" = 21.0;