What is the call sign of the station with the frequency of 90.3mhz
CREATE TABLE "sbn_mom_s_radio_stations" (
  "branding" text,
  "callsign" text,
  "frequency" text,
  "power_k_w" text,
  "location" text
);
SELECT "callsign" FROM "sbn_mom_s_radio_stations" WHERE "frequency"='90.3MHz';