Passage Two
29.
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.
30.
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.
31.
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.

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