What is the average year that anna thompson had an 8th place result in team competition at the world cross country championships in st etienne, france with
CREATE TABLE "achievements" (
  "year" real,
  "tournament" text,
  "venue" text,
  "result" text,
  "extra" text
);
SELECT AVG("year") FROM "achievements" WHERE "tournament"='world cross country championships' AND "venue"='st etienne, france' AND "extra"='team competition' AND "result"='8th';