what is the time when laps is less than 21, manufacturer is aprilia, grid is less than 17 and the rider is thomas luthi?
CREATE TABLE "250cc_classification" (
  "rider" text,
  "manufacturer" text,
  "laps" real,
  "time" text,
  "grid" real
);
SELECT "time" FROM "250cc_classification" WHERE "laps"<21 AND "manufacturer"='aprilia' AND "grid"<17 AND "rider"='thomas luthi';