What is the FIS Nordic World Ski Championship years when the winter Olympics took place in 1968?
CREATE TABLE "men_s_15_km" (
  "winner" text,
  "country" text,
  "winter_olympics" real,
  "fis_nordic_world_ski_championships" real,
  "holmenkollen" text
);
SELECT "fis_nordic_world_ski_championships" FROM "men_s_15_km" WHERE "winter_olympics"=1968;