What is the average Season when the venue was circuit de nevers magny-cours, and Drivers was more than 30?
CREATE TABLE "champions" (
  "season" real,
  "venue" text,
  "teams" real,
  "drivers" real,
  "champion" text,
  "second" text,
  "third" text
);
SELECT AVG("season") FROM "champions" WHERE "venue"='circuit de nevers magny-cours' AND "drivers">30;