[单项选择]The range in frequencies of musical sounds is approximately 20-20,000 cycles per second (cy/sec). Some people can hear higher frequencies khan others. Longitudinal waves whose frequencies are higher than those within the audible range are called ultrasonic frequencies. Ultrasonic frequencies are used in sonar for such purposes as submarine detection and depth finding. Ultrasonic frequencies are also being tried for sterilizing food since these frequencies kill some bacteria. Sound waves of all frequencies in the audible range travel at the same speed in the same medium. In the audible range, the higher the frequency of the sound the higher is the pitch. The term supersonic refers to speed greater than sound. An airplane traveling at supersonic speed is moving at a speed greater than the speed of sound in air at that temperature. Mach 1 means a speed equal to that of sound. Mach 2 means a speed equal to twice that of sound, etc.
Musical sounds have three basic characteristics: pitc
A. inaudible
B. excessively fast
C. characterized by a great amplitude
D. death rays

While other animals use a limited range of sounds or signals to communicate, humans have developed complex systems of language that are used to ensure survival, to express ideas and emotions, to tell stories and remember the past. and to negotiate with one another. Oral (spoken) language is a feature of every human society or culture. Experts studying ancient cultures have several theories about how human language began and developed. The earliest language systems probably combined vocal(声音的) sounds with hand or body signals to express messages. Some words may be similar to natural sounds. Others may have come from expressions of emotion, such as laughter or crying. Language, some theorists believe, is a result of group activities, such as working together or dancing.
Over 6000 languages and major dialects are spoken in the world today. As some languages grow, others disappear. Languages that grow also develop and change due to class, profession, age group, and ot
A. They combined vocal sounds with hand or body signals to express messages.
B. Some words may be similar to natural sounds.
C. Others may have come from expressions of emotion, such as laughter or crying.
D. The language, some theorists believe, is a result of individual activities, such as working together or dancing.