CREATE TABLE table_name_88 (nation VARCHAR, world_rank VARCHAR, birth_date VARCHAR)

what is the nation when the world rank is 6 and the birth date is 1971-07-31?
SELECT nation FROM table_name_88 WHERE world_rank = "6" AND birth_date = "1971-07-31"
SELECT "nation" FROM "table_name_88" WHERE "1971-07-31" = "birth_date" AND "6" = "world_rank"