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
单项选择题 All echo-sounders can measure the().
单项选择题 Aboard ship,vertical flat plates running transversely and connecting the vertical keel to the margin plates are called().
单项选择题 A wooden float placed between a ship and a dock to prevent damage to both is called a().