What opposition do 31.1% of Social Democrats have?
CREATE TABLE "opinion_polls" (
  "polling_firm" text,
  "date" text,
  "venstre_v" text,
  "social_democrats_a" text,
  "danish_people_s_party_o" text,
  "socialist_people_s_party_f" text,
  "conservative_people_s_party_c" text,
  "social_liberal_party_b" text,
  "liberal_alliance_i" text,
  "red_green_alliance" text,
  "christian_democrats_k" text,
  "government" text,
  "opposition" text
);
SELECT "opposition" FROM "opinion_polls" WHERE "social_democrats_a"='31.1%';