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

### Instruction:
what is the nation when the world rank is 6 and the birth date is 1971-07-31?

### Inputs:
table_name: table, columns: ['Nation', 'Birth date', 'Height (m)', 'Weight (kg)', 'World Rank']

### Response:
SELECT Nation FROM table WHERE World Rank = 6 AND Birth date = 1971-07-31