user: ### NOW ANSWER:
Question: what is the nation when the world rank is 6 and the birth date is 1971-07-31?
Columns: ['Nation', 'Birth date', 'Height (m)', 'Weight (kg)', 'World Rank']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Australia (AUS)', '1958-04-14', '1.83', '110', '9']
SQL:

assistant: SELECT "Nation" FROM "Table" WHERE "World Rank" = '6' AND "Birth date" = '1971-07-31';