Section B
Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C and D. You should decide on the best choice.
How do you send a message to a submerged submarine, particularly one carrying mi Water may not look like a barrier to communications, but appearances are deceiving. Water strongly absorbs all electro magnetic waves except blue green light and extremely low frequency radio waves. The very low frequency waves now used to contract submarines penetrates only a short distance into the ocean, so tile craft must either surface or send up and antenna(天线) to receive messages, thereby increasing its weakness. A la ser system—accurate over long distances and capable of carrying more data than tile very low frequency waves—would talk to submarine at their normal depths.
In the system, a very broad beam spreading out freely in all directions would be scanned(扫描) over thousands of square miles of ocean so that it wouldn't endanger boats, birds or fish—or the submarines it is supposed to reach.
Since only a small fraction of the laser system will make its way through the air and ocean, receivers mounted on the submarines must be able not only to detect the laser but also to discriminate between it and sunlight. So, military scientists are now working hard on special filter that allow through only the precise wavelengths emitted by the laser. The filtered light, when transformed into electrical signal, can then be decoded. Military planners are confident that laser communication with submarines is feasible.
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.