Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What is the 1991 when 1997 is 18?

### Inputs:
table_name: table, columns: ['Tournament', '2000', '1999', '1998', '1997', '1996', '1995', '1994', '1993', '1992', '1991']

### Response:
SELECT 1991 FROM table WHERE 1997 = 18