B．they enjoy traveling with the sheep
C．they depend on the sheep for existence
D．they find the position most comfortable
62．The underlined word "they" in the last sentence of the first paragraph refers to ______________.
A．birds and parasites B．birds and sheep
C．parasites and sheep D．sheep, birds and parasites
63．We learn from the text that corals depend on plants for _____________.
A．comfort B．light C．food D．oxygen
64．What does the second paragraph mainly discuss?
A．Some animals and plants depend on each other for existence.
B．Some animals and plants develop their relationship easily.
C．Some plants depend on each other for food.
D．Some animals live better together.
C
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 loudspeakers can reduce the unwanted noise.
Physicists have known about the technique for a long time. Sound is made up of pressure waves in the air. If two sound waves of the same frequency (频率) 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 (周期), we can kill the unwanted noise. Using this technique many car makers are racing to develop noise-killing systems (系统) 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 bum 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 car makers haven' t decided if they will put it into production because it would add several hundred dollars to the cost of their cars.
65．Which of the following gives a general idea of how the noise-killing system works?
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.
66．Besides its main use the noise-killing system can __________.
A．make a car lighter
B．make a car quieter
C．reduce the cost of a silencer
D．improve the performance of a silencer
67．The noise-killing system is not yet popular mainly because ___________.