user: Question: What's the highest Date listed that has U-boats destroyed (Pola) of 1?
Columns: ['Date', 'Ships sunk (Pola)', 'Tonnage', 'U-boats destroyed (KuK)', 'U-boats destroyed (Pola)']
Types: ['real', 'text', 'text', 'text', 'text']
Sample row: [1914.0, 'n/a', 'n/a', 'none', 'n/a']
SQL:

assistant: SELECT MAX("Date") FROM "Table" WHERE "U-boats destroyed (Pola)" = '1';