填空题

When must the sonar devices be placed
Before the ______.

【参考答案】

undersea vehicle dives
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Doves flying in daylight use the sun for guidance. But the stars are obviously their most important means of navigation. They find their way by such landmarks as mountains ranges, coastlines, and river courses. When it is too dark to see these, the doves circle helplessly, unable to get their bearings.
填空题
Why is ocean exploration not so expensive as space travel Because ______ for submersibles to get away from the Earth’s gravity.
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