What is the Engine(s) that has  f Tyres in 1971?
CREATE TABLE "complete_world_championship_formula_one_" (
  "year" real,
  "chassis" text,
  "engine_s" text,
  "tyres" text,
  "points" text
);
SELECT "engine_s" FROM "complete_world_championship_formula_one_" WHERE "tyres"='f' AND "year"=1971;