[单项选择] 第二篇 The Magic of Sound Music is one of the most beautiful forms of artistic expressions ever invented. In movies and plays, music has an added function: it not only moves people but also can shock people. Our eardrums can withstand sound within 20 to 80 decibels(分贝). Once sound exceeds this limit, even beautiful music will become ear-splitting noise and harm health.A strong blast(响声) of high sound can twist and break a solid iron sheet. High sound of 150 decibels can kill a healthy rat. In movies, sometimes the hero can produce a sound that ordinary people can’t hear and only those who have the same ability can feel.In nature, there is actually sound that is beyond our hearing. In physics, the sound that exceeds 20000Hz is called ultrasonic(超音波的). Dolphins, whales and bats can make such high-frequency sound. It does no harm to health. Sound less than 20Hz is called infrasonic(次声)waves. When we move, the air will vibrate. The vibration o
A. It may be harmful to people’s health
B. It always cheers people up
C. It is very often difficult to understand
D. It sounds better when it is loud enough