What year was the structure in greensboro, north carolina considered tallest?
CREATE TABLE "structures_past_or_present_between_550_a" (
  "pinnacle_height" text,
  "year" real,
  "structure_type" text,
  "main_use" text,
  "country" text,
  "town" text
);
SELECT "year" FROM "structures_past_or_present_between_550_a" WHERE "town"='greensboro, north carolina';