单项选择题

The returning echoes the receiver picks up are ______.

A．dearly audible bleeps
B．inaudible to human ears
C．immediately passed on to headphones
D．filtered through the transmitter
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单项选择题
The attempt to help blind people to see with sound ______.
A．was similar to the sound location system of the bats
B．led to a product that sent out sound waves of about 20,000 cycles per second
C．was experimented on a submarine
D．turned out to be a failure
单项选择题
The fourth paragraph of the passage primarily discusses ______.
A．the reproduction of sea cucumbers
B．the food sources of sea cucumbers
C．the eating habits of sea cucumbers
D．threats to sea cucumbers' existence
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