What's the frequency when the identifier is cbf-fm-9?
CREATE TABLE "s_rebroadcaster_of_cbf_fm" (
  "city_of_license" text,
  "identifier" text,
  "frequency" text,
  "power" text,
  "class" text,
  "rec_net" text
);
SELECT "frequency" FROM "s_rebroadcaster_of_cbf_fm" WHERE "identifier"='cbf-fm-9';