Why didn’t people begin to use unmanned vehicles in undersea exploration until recently? It is because of the ______. Why is o

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问题 Why didn’t people begin to use unmanned vehicles in undersea exploration until recently?
It is because of the ______.

Why is ocean exploration not so expensive as space travel?
Because ______ for submersibles to get away from the Earth’s gravity.

选项

答案there’s no need

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