People did not begin to use unmanned vehicles in undersea exploration until recently because of______. Ocean exploration is not

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问题 People did not begin to use unmanned vehicles in undersea exploration until recently because of______.
Ocean exploration is not so expensive as space travel mainly because ______.

选项 A、it is not affected 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

答案D

解析 参阅第六段第一句,escape Earth’s gravity与本题的正确选项 get away from Earth’s gravity相关。
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