In which round did Roy Salvadori won Class D and Alan Hutcheson won Class B?
CREATE TABLE "calendar_winners" (
  "round" real,
  "circuit" text,
  "date" text,
  "class_a_winner" text,
  "class_b_winner" text,
  "class_c_winner" text,
  "class_d_winner" text
);
SELECT "round" FROM "calendar_winners" WHERE "class_d_winner"='Roy Salvadori' AND "class_b_winner"='Alan Hutcheson';