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
What can be inferred about the defence mechanisms of the sea cucumber
A．They are very sensitive to surrounding stimuli.
B．They are almost useless.
C．They require group cooperation.
D．They are similar to those of most sea creatures.
单项选择题
Which of the following would NOT cause a sea cucumber to release its internal organs into the water
A．A touch.
B．Food.
C．Unusually warm water
D．Pollution.
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