单项选择题

The sonar devices must be placed _________.

A．from time to time
B．after the undersea vehicle dives
C．at the time when the undersea vehicle is diving
D．before the undersea vehicle dives
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单项选择题
People did not begin to use unmanned vehicles in undersea exploration until recently because of _________.
A．the communication problem
B．the ocean depths
C．the movement of waves
D．the problem of vacuum
单项选择题
By more challenging (Para. 6, Filet sentence), the author means _________.
A．more interesting
B．more demanding
C．more attractive
D．more dangerous
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