单项选择题

The reference word "it" ( Sentence 1 , Para. 3 ) refers to ______.

A．the laser beam
B．the air C) the ocean
C．the submarine
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单项选择题
Which of the following is TRUE according to the passage
A．Water is a barrier to radio communication.
B．Lasers have found wide application in submarine communication.
C．Water absorbs all kinds of electromagnetic waves.
D．Very low frequency radio waves cannot be used to contact submarines.
单项选择题
Which of the following does the passage mainly discuss
A．Missiles carried by submerged submarines.
B．Messages sent by submerged submarines.
C．Blue-green lasers used by submerged submarines.
D．The way to send a message to submerged submarines.
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