Which Tonnage (GRT) is the highest one that has a Date of 16 june 1940?
CREATE TABLE "ships_attacked" (
  "date" text,
  "u_boat" text,
  "name" text,
  "nationality" text,
  "tonnage_grt" real,
  "fate" text
);
SELECT MAX("tonnage_grt") FROM "ships_attacked" WHERE "date"='16 june 1940';