A、They don’t need conventional signs and symbols. B、They only require a receiver in communication. C、The distance between commun

Under normal conditions the act of communication requires the presence of at least two persons: one who sends and one who receives the communication. In order to communicate thoughts and feelings, there must be a conventional system of signs or symbols which mean the same to the sender and the receiver. The means of sending communications are too numerous and varied for systematic classification; therefore, the analysis must begin with the means of receiving communications.
    Reception of communication is achieved by our senses. Sight, hearing and touch play the most important roles. Smell and taste play very limited roles. Examples of visual communication are gesture and imitation. Although both frequently accompany speech, there are systems that rely solely on sight, such as those used by deaf and dumb persons. Another means of communicating visually is by signals of fire, smoke, flags or flashing lights. Feelings may be simply communicated by touch such as by handshaking, although a highly-developed system of handshaking disables blind, deaf, and dumb persons to communicate intelligently. Whistling to someone, clapping hands in a theater, and other forms of communication by sound rely upon the ear as a receiver. The most fully-developed form of auditory communication is, of course, the spoken language.
    The means of communication mentioned so far have two features in common: they last only a short time, and the persons involved must be relatively close to each other. Therefore, all are restricted in time and space.