user: Question: Which class has a year prior to 2011 and Audi R15 TDI as the chassis?
Columns: ['Year', 'Entrant', 'Class', 'Chassis', 'Engine', 'Rank', 'Points']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1999.0, 'Champion Racing', 'LMP', 'Porsche 911 GT1 Evo', 'Porsche 3.2L Turbo Flat-6', '25th', '47']
SQL:

assistant: SELECT "Class" FROM "Table" WHERE "Year" < 2011.0 AND "Chassis" = 'Audi R15 TDI';