单项选择题
A drilling bit which is doughnut shaped to permit recovery of the center portion of the hole drilled is called a（）.
A.diverter bit
B.hole reamer
C.core bit
D.conventional bit
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A drawbridge may use visual signals to acknowledge a vessel’s request to open the draw. Which signal indicates that the draw will NOT be opened immediately?（）
A.A flashing amber light
B.A fixed red light
C.A white flag raised and lowered vertically
D.A flashing white light -
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A Doppler speed log indicates speed through the water（）.
A.at all times
B.in the bottom return mode
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D.only when there is no current -
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A disadvantage of using chain lashing on heavy vehicles aboard Ro-Ro vessels is that it is （）.
A.heavy
B.easily damaged
C.affected by temperature
D.costly relative to the strength ratio
