user: Question: What episode aired on 18 April 2007?
Columns: ['#', 'Air Date', 'Challenge', 'Winner', 'Test-taker', 'Passed?']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, '18 April 2007', 'Pace Car Challenge', 'Red', 'N/A', 'N/A']
SQL:

assistant: SELECT "#" FROM "Table" WHERE "Air Date" = '18 April 2007';