In which year did Blount County build a bridge in Cleveland that was listed on 1981-08-20?
CREATE TABLE "references" (
  "name" text,
  "built" text,
  "listed" text,
  "location" text,
  "county" text
);
SELECT "built" FROM "references" WHERE "county"='blount' AND "listed"='1981-08-20' AND "location"='cleveland';