单项选择题

The returning echoes the receiver picks up are ________.

A．clearly 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
Which of the following is NOT true about the ultrasonic torch
A．It enables the blind to live a more normal life.
B．It is mainly used to scan the area in front of the operator for obstacles.
C．Fast succession of bleeps indicates that the obstacle is near.
D．Its sound can warn other people of the blind man’s approach.
单项选择题
According to the passage, the Pony Express riders are ________.
A．orphans
B．well over 18
C．skillful can-drivers
D．courageous and brave young men
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