Passage Two
Objects that are vibrating make sounds. Rhythmic sound vibrations produce sound waves which travel in every direction. The frequency of a sound is the number of waves that pass a given point each second. [29]The more rapidly an 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. [30]Humans hear sounds that have frequencies from about 20 hertz to 20,000 hertz. Sound with a frequency higher than 20,000 hertz is called ultrasound which 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 and then listen to the echoes bouncing off things like insects. The shorter the time interval between the squeak and the echo, the closer they are to the insect. [31]Using sound to find or "see" things is called echolocation. 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. [31]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 away.

Questions 29 to 31 are based on the passage you have just heard.
29. What is the relationship between the vibration of an object and the frequency of its sound?
正确答案：A
解析：细节题。短文提到，物体振动得越快，它制造的声音频率就越高。A为原文中the more...the greater结构的同义表达，故为正确答案。B与C意思相同，可同时排除；D明显与原文内容相冲突。
30. What sounds can humans hear?
正确答案：C
解析：细节题。文中提到，人类可以听见频率在20赫兹到2万赫兹之间的声音。由此可知，C为正确答案。
31. According to the speaker, what is the phenomenon of echolocation?
正确答案：B
解析：细节题。文中提到，用声音来找或看东西被称为“回声定位法”。之后，短文以海豚为例说明“回声定位法”：海豚会发出尖利的声音，这种声音会撞击到它周围的任何物体，然后通过声波形式再传回到海豚处。通过这种方法，海豚可以找到离自己10英尺以内的小鱼。蝙蝠只是利用回声定位原理“找到”食物，但并非caught或ate，所以可排除强干扰项A。