What is the surface when the us open is the championship in the year 1979?
CREATE TABLE "doubles_37_31_6" (
  "outcome" text,
  "year" real,
  "championship" text,
  "surface" text,
  "partner" text,
  "opponents" text,
  "score" text
);
SELECT "surface" FROM "doubles_37_31_6" WHERE "championship"='US Open' AND "year"=1979;