填空题

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

【参考答案】

communication problem

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填空题

When must the sonar devices be placed Before the ______.

问答题

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.

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