What are the major themes of "Barn Burning"？

B．Major industries

C．Cars and population

D．Gas fumes and the burning for energy

A、poor

B、poverty-stricken

C、wealthy

D、rich

To fight a noise by making more noise sounds strange, but that' s exactly what motor engi-neers are doing in Germany and some other countries．

Carmaker' s research and development laboratories have already proved that mixing more noisewith the help of loudspeakers can reduce the unwanted noise．Physicists have known about thetechnique for a long time．Sound is made up of pressure waves in the air．If two sound waves ofthe same frequency(频率) mix so that the highest point of one wave happens at the same time asthe lowest point of the other wave, the result is no sound．Therefore, by producing a perfect copyof the noise and delaying it by half a wave cycle (周期), we can kill the unwanted noise．Usingthis technique many carmakers are racing to develop noise killing systems (系统) both inside andoutside the cars．

Another good thing about the use of noise killing systems is that it saves the need for a silencer;this not only reduces the weight of a car, but also makes the motor burn less fuel and work better．

Some engineers believe that the noise killing system will be used in most cars within a fewyears．But the carmakers haven' t decided if they will carry it into production because it would addseveral hundred dollars to the cost of their cars．

Which of the following gives a general idea of how the noise killing system works？ 查看材料

A.By producing pressure waves going in the opposite direction．

B.By mixing high frequency sound waves with low frequency sound waves．

C.By mixing new sound waves with the noise and sending them out together．

D.By making copies of the unwanted sound waves and letting them out a little later．

A、concave lens

B、cylinder lens

C、convex lens

D、plane lens

A、Temperature

B、Moisture content

C、pH of the wood

D、Presence of natural toxins

A、a painful one

B、a happy one

C、a busy one

D、a tough one

A、writing and exercising

B、hunting and boxing

C、sports and writing

D、hunting and fishing