Which European Cup is in the 1990-91 season?
CREATE TABLE "all_the_triple_crowns_achieved" (
  "season" text,
  "club" text,
  "national_league" text,
  "national_cup" text,
  "european_cup" text
);
SELECT "european_cup" FROM "all_the_triple_crowns_achieved" WHERE "season"='1990-91';