The symbol phenomenon refers to the two things A and B that have no correlation in ideology. When we use the A thing to represent the B thing, the A thing can be regarded as the symbol of the B thing. According to the above definitions, which of the following is not a symbol phenomenon? ['Siren of the fire truck.', 'Cross marks used by medical institutions.', 'Scales carved on the court gate.', "The referee's whistle."]