Have you ever heard an echo of your voice?
An echo is a reflected(反射)sound. Stand in a very large, empty hall with nothing on the walls. Then shout. The sounds you make will hit the walls and come back to you. The walls reflect the sounds. These reflected sounds are called “echoes”.
We can use reflected sound to make sound travel farther. Hold a watch about fifty centimeters from your ear. You cannot hear it. The sound of the watch travels a short way in all directions. It does not travel as far as your ear. You can make it travel to your ear by using a roll of cardboard(纸板筒).
Make the roll of cardboard about fifty centimeters long. Put one end over your ear and hold the watch at the other. Now you can hear the watch because the sound has not gone in all directions. It is reflected down the roll to your ear.
A megaphone(喇叭筒)works in the same way. It makes sound travel farther. It reflects the sound into a beam which can travel a long way. In the ship, the sailor uses the megaphone to make his voice travel. Without the megaphone, his voice cannot travel far.
We use reflected sound in many ways. We use it to measure the depth of the sea, for example. Among the animals, bats and whales use reflected sound to tell them where they are and what things are near them.
Which is the right order of hearing our echo in a very large, empty hall?
a. We hear the reflected sound       b. The sound returns to us  
c. The sound hits the walls          d. We make a sound

A．c, b, a, d

B．c, a, b, d

C．d, c, b, a

D．d, b, c, a

. From the text we learn that __________ .

A．reflected sound is useful only to blind people

B．people haven’t made use of reflected sound yet

C．bats use reflected sound to fly and get food

D．whales don’t use reflected sound as they live in the sea

If you haven’t got a megaphone, ____ will help you to make your voice travel farther.

A．a long piece of wood	B．a roll of cardboard
C．a thin thread	D．a long piece of silk