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  3. As in the field of space travel, so in undersea exploration new technologies continue to appear. They share a number of similari

As in the field of space travel, so in undersea exploration new technologies continue to appear. They share a number of similari

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问题    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, like spaceships, must maintain living conditions in an unnatural environment. But 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 the 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 re- mote-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.
   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 aspects, 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.
People did not begin to use unmanned vehicles in undersea exploration until recently because of______.
选项 A、the communication problem
B、the movement of waves
C、the ocean depths
D、the problem of vacuum
答案A
解析 就为什么“人们直到最近才开始在海底探索中使用无人驾驶的遥控装置”这一问题，作者在第四段开头和第五段开头明确地给出了原因：“one reason for this is communicating with vehicles in orbit is much easier than talking to those underwater”及“For a similar reason, knowing where you are undersea is much more difficult than in space”。因此可以断定[A]是正确答案。
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