Just as in the field of space travel, so in undersea exploration new technologies continue to appear. 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 of 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. 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.
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 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. People did not begin to use unmanned vehicles in undersea exploration until recently because of          .
A) the communication problem                C) the movement of waves
B) the ocean depths                         D) the problem of vacuum

2. The sonar devices must be placed         .
A) from time to time
B) before the undersea vehicle dives
C) at the time when the undersea vehicle is diving生活日记网(www．Examw。com)
D) after the undersea vehicle dives

3. By "more challenging" the author means         .
A) more interesting                         C) more attractive
B) more dangerous                         D) more demanding

4. Ocean exploration is not so expensive as space travel mainly because         .
A) it is not affected by Earth’s gravity
B) most undersea vehicles operate at the end of long ropes
C) ROVs have been put to use
D) There is no need for submersibles to get away from Earth’s gravity

5. Which of the following is NOT MENTIONED in the passage?
A) New techniques are continually developed and new machines invented for undersea exploration
B) If not well built, a submersible is most likely to get crushed in deep water.
C) A submersible, when manned, looks very much like a spaceship.
D) In certain aspects undersea exploration is often faced with the same problem as space travel is.

参考答案：Passage 4.        A       B       D      D      C

像太空探险领域一样，海底探险领域的新技术不断涌现。太空旅行和海洋探险二者既有很多共性，同时也有重大差异。
像宇宙飞船一样，载人潜水器必须在非自然环境中维持生存条件。在太空的真空状态下，宇宙飞船必须做到密封，而处于深海压力中的潜艇要能承载极大压力，否则会被挤压成碎片。太空探险领域，无人飞船先于载人飞船升空，而在海洋探险领域载人潜艇先于无人潜艇潜入海底，无人遥控潜艇最近才投入应用。真空是无线电通信的理想媒介，但水下通信仅限于很短的声波，这样，多数潜水器运行时都要拖着长长的电缆。
由于同样的原因，水下定位也比太空定位困难的多。对宇宙飞船的定位可以通过跟踪无线电信号完成，也可以借助望远镜或雷达完成。然而，对海底探险来说，必须要事先将专用的声纳系统放置在潜水区域才能完成对潜艇的定位。
尽管海洋探险在很多方面比太空探险有更多的要求，但是海洋探险自身有明显的优点：潜入海底不需要外力摆脱地球引力。因而海洋探险要廉价的多。