Tell me the class for admiral and hms frobisher
CREATE TABLE "ships_served_in_by_richard_bolitho" (
  "ship" text,
  "guns" text,
  "class" text,
  "captain" text,
  "rank" text,
  "year" text
);
SELECT "class" FROM "ships_served_in_by_richard_bolitho" WHERE "rank"='admiral' AND "ship"='hms frobisher';