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 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
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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