Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.

          Although speech is the most advanced form of communication, there are many ways of communicating without using speech. Signals, signs, symbols, and gestures may be found in every known culture. The basic function of signal is to impinge upon the environment in such a way that it attracts attention, as, for example, the dots and dashes of a telegraph circuit. Coded to refer to speech, the potential for communication is very great. Less adaptable to the codification of words, signs also contain meaning in and of themselves. A stop sign or a barber pole conveys meaning quickly and conveniently.  

          Symbols are more difficult to describe than either signals or signs because of their intricate relationship with the receiver’s cultural perceptions. In some cultures, applauding in a theater provides performers with an auditory symbol of approval. Gestures such as waving and handshaking also communicate certain cultural messages.

          Although signals, signs, symbols, and gestures are very useful, they do have a major disadvantage in communication. They usually do not allow ideas to be shared without the sender being directly adjacent to the receiver. Without an exchange of ideas, interaction comes to a halt. As a result, means of communication intended to be used for long distances and extended periods must be based upon speech. To radio, television, and the telephone, one must add fax, paging systems, electronic mail, and the Internet, and no one doubts but that there are more means of communication on the horizon.

According to this passage, what is a signal?