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  3. Why didn’t people begin to use unmanned vehicles in undersea exploration until r

Why didn’t people begin to use unmanned vehicles in undersea exploration until r

游客2023-10-03  55
问题 Why didn’t people begin to use unmanned vehicles in undersea exploration until recently?
   It is because of the ______.
[br] What does the author mean by saying "more challenging" in the last paragraph?
选项
答案 More demanding
解析
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