CREATE TABLE table_name_20 (time VARCHAR(255), rider VARCHAR(255), grid VARCHAR(255), laps VARCHAR(255), manufacturer VARCHAR(255))
what is the time when laps is less than 21, manufacturer is aprilia, grid is less than 17 and the rider is thomas luthi?
    
SQL: SELECT time FROM table_name_20 WHERE laps < 21 AND manufacturer = "aprilia" AND grid < 17 AND rider = "thomas luthi"