user: Question: Which Week has an Opponent of San Francisco 49ers?
Columns: ['Week', 'Date', 'Opponent', 'Result', 'Attendance']
Types: ['real', 'text', 'text', 'text', 'real']
Sample row: [1.0, 'September 29, 1957', 'Philadelphia Eagles', 'W 17-13', 62506.0]
SQL:

assistant: SELECT "Week" FROM "Table" WHERE "Opponent" = 'San Francisco 49ers';