单项选择题
All electrical appliances aboard a vessel should be grounded to（）.
A.prevent them from falling when the vessel rolls
B.protect personnel from electrical shock
C.increase their operating efficiency
D.prevent unauthorized personnel from operating them
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All echo-sounders can measure the（）.
A.actual depth of water
B.actual depth of water below keel
C.average depth from waterline to hard bottom
D.average depth of water to soft bottom -
单项选择题
Aboard ship，vertical flat plates running transversely and connecting the vertical keel to the margin plates are called（）.
A.floors
B.intercostals
C.girders
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A wooden float placed between a ship and a dock to prevent damage to both is called a（）.
A.camel
B.dolphin
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