user: Question: What date was the U21 pennant laid down?
Columns: ['Name', 'Pennant', 'Builder', 'Laid Down', 'Launched', 'Commissioned']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Sutlej', 'U95', 'Denny , Dunbarton', '4 January 1940', '1 October 1940', '23 April 1941']
SQL:

assistant: SELECT "Laid Down" FROM "Table" WHERE "Pennant" = 'U21';