user: Question: Who was the opposing team in the game with a score of 21-17?
Columns: ['Date', 'Bowl Game', 'Big Ten Team', 'Opp. Team', 'Score']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Dec. 26, 2007', 'Motor City Bowl', 'Purdue', 'Central Michigan', '51-48']
SQL:

assistant: SELECT "Opp. Team" FROM "Table" WHERE "Score" = '21-17';