A、 The quicker it vibrates, the greater the frequency of sound it produces.
B、 The slower it vibrates, the higher the frequency of sound it produces.
C、 The quicker it vibrates, the lower the frequency of sound it produces.
D、 The frequency of sound it produces is not in proportion to the speed it vibrates.

【正确答案】 A

【答案解析】解析：短文提到，物体振动得越快，它制造的声音频率就越高。A为录音原文中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。

A、 Those with frequencies lower than 20 hertz.
B、 Those with frequencies about 21,000 hertz.
C、 Those with frequencies lower than 20,000 hertz.
D、 Those with frequencies higher than 20,000 hertz.

【正确答案】 C

【答案解析】解析：录音提到，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。

A、 A bat caught the insect in midair and ate it.
B、 Dolphin's clicking sounds bounced off the fish and back to it.
C、 A person heard an echo when he shouted into the valley.
D、 A dog heard his owner's whistle because he used ultrasound.

【正确答案】 B

【答案解析】解析：细节题。录音提到，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。