user: Question: What's the rank when attendance was less than 18073 and having Northwestern as an opponent?
Columns: ['Rank', 'Attendance', 'Opponent', 'Result', 'Date']
Types: ['real', 'real', 'text', 'text', 'text']
Sample row: [1.0, 18735.0, 'Wichita State', 'L 68–89', 'February 11, 2012']
SQL:

assistant: SELECT "Rank" FROM "Table" WHERE "Attendance" < 18073.0 AND "Opponent" = 'Northwestern';