D 
Seeing with Sound 
    Have you ever watched a dog lift up its ears and bark at the air? That dog has just heard a sound that is too high-pitched(声调高的)for you to hear. 
　　The Science of Sound 
　　Objects that are vibrating(振动)，moving back and forth quickly, make sounds. Rhythmic sound vibrations produce sound waves. Sound waves travel in every direction. The frequency of a sound is the number of waves that pass a given point each second. The more rapidly and object vibrates, the greater the frequency of sound it makes. Frequency is measured in hertz. One hertz equals one sound wave per second. 
 　  The frequency of sound determines the degree of highness or lowness of the sound. A high-pitched sound has a higher frequency than a low-pitched sound. Humans hear sounds that have frequencies from about 20 hertz to 20,000 hertz.. 
　　Seeing with Sound 
　　Sound with a frequency higher than 20,000 hertz is called ultrasound. In other words, ultrasound is too high-pitched for humans to hear. However, many animals can hear ultrasound. Bats can hear higher sounds than any other animal. Some can hear up to 210,000 hertz. Bats send out lots of high-pitched squeaks(吱吱叫声)and then listen to the echoes bouncing(弹跳)off things like insects. The shorter the time interval between the squeak and the echo, the closer they are to the insect. 　　　Using sound to find or “see” things is called echolocation. 
　　Dolphin Sounds 
　　Another animal that uses ultrasound is the dolphin. Dolphins can hear sounds up to about 150,000 hertz. Dolphins use sound waves to find food underwater. They make clicking sounds that hit whatever is around them and then bounce back to the dolphins. Dolphins can send as many as 2,000 clicks per second. Dolphins’ ultrasound ability is so sensitive that they can detect small fish from 10 feet (3m) away. 
Now what about that dog that heard a sound that you did not? The sound was his owner using an ultrasonic whistle to call him home. Because dogs can hear up to 50,000 hertz, the dog at the park will head home. 

68. To be able to hear ultrasound, you must be able to hear______. 
A. less than 20 hertz            
B. exactly 20,000 hertz 
C. less than 20,000 hertz        
D. more than 20,000 hertz 

69. Which animal can hear the highest ultrasound according to the passage? 
A. A bat.        B. A dog.        C. A dolphin.         D. A human. 

70. Which of the following statements is an example of echolocation? 
A. The bat caught the insect in midair and flew off to eat it. 
B. The dolphin made clicking sounds that bounced off the fish and back to it. 
C. The person heard an echo when he shouted into the valley. 
D. The dog heard his owner’s ultrasonic whistle because he used ultrasound.

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