CREATE TABLE "defunct_callsigns" (
  "callsign" text,
  "area_served" text,
  "frequency" text,
  "band" text,
  "on_air_id" text,
  "purpose" text
);

Question: what is the callsign whtat uses 7bu?
SQL: SELECT "callsign" FROM "defunct_callsigns" WHERE "on_air_id"='7bu';