The world is filled with many interesting sounds. Some are very pleasant to hear; others are unpleasant to our ears. In a day, you probably hear hundreds of different sounds.
All sounds are different. Some may be soft; others may be loud. Some sounds are high; others are low.
Some sounds are useful. Without sound we cannot talk or listen to one another. The ringing alarm clock (闹钟)makes people wake up. The hooting of a horn (喇叭声) on the road warns careless people of danger.
We make sounds by making things move to and fro(来回地). This to-and-fro movement is called vibration (振动). It means moving up and down or forwards(向前地) and backwards(向后的) very fast. This makes sound waves(波).
We can make sounds in many ways, for example, by shouting, clapping our hands and striking the table. We can make sounds only by making things vibrate. We can talk because we can make our vocal cords (声带) vibrate. Our vocal cords are in a sound box or voice box. Two vocal cords vibrate when our lungs(肺) force the air through them.
We can make sounds ______.

A．by making things still

B．by making things vibrate

C．by making things jump up and down

D．by changing the positions of things

Some sounds are useful because ______.

A．they can wake people up

B．they can warn people to be careful when they are crossing the road

C．they make it possible for people to talk or listen to each other

D．all of the above

When the air is through our vocal cords, it can make them ______.

A．vibrate	B．breathe the air
C．talk to each other	D．move forward and backward

Sounds are different because ______.

A．different sounds have different vibrations

B．different sounds have different sound waves

C．different sounds have different vocal cords

D．both A and B