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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.(潜水器), 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 deep water.
    In exploring space, unmanned vehicles were employed before astronauts. In undersea exploration, on the other haled, men paved (铺路) 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 process.
    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) devices most 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 doesn’t require the power necessary to escape Earth’s gravity. Thus, it remains far less expensive.
Ocean exploration is not so expensive as space travel mainly because ______.
选项 A、most undersea vehicles operate at tile end of long ropes
B、ROVs have been put to use
C、it is not affected by Earth’s gravity
D、there is no need for submersibles to get away from Earth’s gravity
答案D
解析 第六段最后两句：潜入洋底不需要脱离地球引力所需的动力，这样花费就少得多
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