user: Question: When 21 is the number in series what is the air date?
Columns: ['No. in series', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code']
Types: ['real', 'text', 'text', 'text', 'text', 'real']
Sample row: [1.0, '"The Mama Who Came to Dinner"', 'Joel Zwick', 'William Bickley & Michael Warren', 'September 22, 1989', 445230.0]
SQL:

assistant: SELECT "Original air date" FROM "Table" WHERE "No. in series" = 21.0;