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?
正確答案：A
解析：細節題。短文提到，物體振動得越快，它制造的聲音頻率就越高。A為原文中the more...the greater結構的同義表達，故為正確答案。B與C意思相同，可同時排除；D明顯與原文內容相沖突。
30. What sounds can humans hear?
正確答案：C
解析：細節題。文中提到，人類可以聽見頻率在20赫茲到2萬赫茲之間的聲音。由此可知，C為正確答案。
31. According to the speaker, what is the phenomenon of echolocation?
正確答案：B
解析：細節題。文中提到，用聲音來找或看東西被稱為“回聲定位法”。之后，短文以海豚為例說明“回聲定位法”：海豚會發出尖利的聲音，這種聲音會撞擊到它周圍的任何物體，然后通過聲波形式再傳回到海豚處。通過這種方法，海豚可以找到離自己10英尺以內的小魚。蝙蝠只是利用回聲定位原理“找到”食物，但并非caught或ate，所以可排除強干擾項A。