根据下面资料，回答40-43题
Life gets noisier every day and very few people can free from noise of some sort or another．It doesn' t matter where you live--in the middle of a modern city, or a faraway village--the chances that you will be disturbed by jet aeroplanes, transistor radios, oil powered engines, etc．, are almost
everywhere．We seem to be getting used to noise, too．Some people feel quite lonely without background music while they are working．
Scientific tests have shown that total silence can be very frightening experience for human beings．
However, some people enjoy listening to pop music which is very loud, and this can do harm to their eardrums(耳鼓)．The noise level in some disco is far above the usual safety level for heavy industrial areas．
One recent report about noise and concentration(专心)suggested that although a lot of people say that any noise disturbs their concentration, what really affects their ability to concentrate is a change in the level of noise．It goes on to say that a background noise, which doesn' t change too much ( music, for example) may even help people to concentrate．
Scientists have discovered that what prevents people from concentrating is_______．
A.any kind of noise
B.great changes in level of noise
C.background noise
D.various background music