阅读理解。

     As in the field of space travel, new technologies continue to appear in undersea exploration. They share a number of similarities with each other - as well as some important differences.

     Manned submersibles (潜水器), like spaceships, must maintain living conditions in an unnatural

environment. While a spaceship must simply be sealed against the vacuum space, a submersible must be

able to bear extreme pressure if it is not to break up in deep water.

     In exploring space, unmanned vehicles were employed before astronauts. In undersea exploration, on

the other hand, men paved the way, and only recently have unmanned remote-operated vehicles (ROVs)

been put to use.

     One reason for this is that communicating with vehicles in orbit is much easier than talking to those

underwater. A vacuum is an ideal medium for radio communications, but underwater communications are

limited to much slower sound waves. Thus, most undersea vehicles - particularly ROVs - operate at the

end of long ropes.(电缆终端)

     For a similar reason, knowing where you are undersea is much more difficult than in space. A

spaceship's position can be located by following its radio signal, or by using telescopes and radar. For an

undersea vehicle, however, a special network of sonar (声纳系统) devices must be laid out in advance on the ocean floor in the area of a dive to locate the vehicle's position.

     Though undersea exploration is more challenging than outer space in a number of respects, it has a

distinct advantage: going to the ocean depths doesn't require the power necessary to escape Earth's

gravity. Thus, it remains far less expensive.

1. The purpose of the passage is _____.

A. to persuade you to explore the depths of the ocean

B. to stress the importance of the undersea exploration

C. to make you believe that the undersea exploration is better

D. to tell some differences between two kinds of explorations  

2. By saying "men paved the way" in Paragraph 2 the author means that in undersea exploration _____.

A. unmanned vehicles were used in the beginning

B. men covered the ocean floor with stones and bricks

C. manned vehicles were employed before unmanned ones      

D. men invented unmanned remote-operated vehicles in the past

3. The sonar devices must be placed _____.

A. from time to time                

B. after the undersea vehicles dive

C. before the undersea vehicles dive

D. when the undersea vehicles are diving

4. What can we infer from the passage?

A. Submersibles usually break up in deep water.

B. Undersea vehicles can receive signals immediately.

C. Going to space needs power to escape the gravity.

D. Radio communications are quite difficult in a vacuum.