The idea of fighting a noise by making more noise sounds strange, but that's exactly what motor engineers are doing in Germany and some other countries. Car makers' research and development laboratories have already proved that mixing in more noise with the help of loudspeaker can reduce the unwanted noise. Physicists have known about the technique for a long time. Sound is made up of pressure waves in tire air. If two sound waves of the same frequency mix so that the highest point of one wave happens at the same time as the lowest point of the other wave, the result is no sound. Therefore, by producing a perfect copy of the noise and delaying it by half a wave cycle, we can kill the unwanted noise. Using this technique many carmakers are racing to develop noise-killing systems both inside and outside 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 few years. But the carmakers haven't decided if they will put it into production because it would add several hundred dollars to the cost of their cars. Which of the following gives a general idea of how the noise-killing system works?