Stop! Listen! What do you hear? You may hear many different sounds. Some of those sounds may be noisy.  Noise is loud or unwanted sound.

     Noise can be caused by many kinds of machines, such as motorcycles, jet planes, tractors and so on. And, to some people, rock bands (摇滚乐队) are also noise.

     What happens to people who live near noisy machines or use them over a period of time? Doctors find that these people have trouble in sleeping. But, most important, constant  (不断的) loud noise can cause a loss of hearing.

     Scientists use a unit (单位) of measure called a decibel  (分贝)  to measure the loudness of a sound. The sound of a quiet room, for example, measures 35 decibels. Talking measures between 40 to 65 decibels. Sounds from traffic and from some rock bands can measure over 120 decibels. Noise at this high level causes the greatest hearing loss.

     49.  From the passage, we know noise is _______.

          A. wanted sound                 B. loud or unwanted sound

          C. all sounds that you can hear      D. comfortable sound

     50. A "decibel" is a _______.

          A. unit that measures sound

          B. kind of noise

          C. machine that makes very loud noise

          D. person who hates noise

     51.  According to the passage, loud noise can_______.

          A. help people sleep better       B. stop machines

          C. cause a loss of hearing        D. kill people

Stop. Listen! What do you hear? You may hear many different sounds. Some of those sounds may be noise. Noise is a loud or unwanted sound.
Noise can be caused by many kinds of machines, such as motorcycles, jet planes, farm tractors(拖拉机), rock music is also noise.
What happens to people who live near noisy machines or use them over a period of time? Doctors have found that these people have trouble in sleeping. But, most important, constant(不断的) loud noise can cause a loss of hearing.
Scientists use a unit of measure called decibel(分贝) to measure the loudness of a sound. The sound of a quiet room, for example, measures 35 decibels. Talking measures between 40 and 65 decibels. Sounds from traffic and from some rock bands can measure over 120 decibels. Noise at this high level(层次) causes the great hearing loss.
根据短文内容，选择正确的答案（每小题1分，共5分）
【小题1】According to the passage, noise is      .