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
According to the passage, why is the shape of sea cucumbers important
A．h helps them to digest their food.
B．It helps them to protect themselves from danger.
C．It makes it easier for them to move through the mud.
D．It makes them attractive to fis
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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