Table schema:
CREATE TABLE table_name_20 (time VARCHAR, rider VARCHAR, grid VARCHAR, laps VARCHAR, manufacturer VARCHAR)

Question: 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"