user: Question: Who was the opponent when 48,121 people attended?
Columns: ['Week', 'Opponent', 'Result', 'Game site', 'Attendance']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['1', 'at San Diego Chargers', 'L 14–21', 'San Diego Stadium', '54,061']
SQL:

assistant: SELECT "Opponent" FROM "Table" WHERE "Attendance" = '48,121';