单选题（）(富余水深) is the depth remaining under a ship’s bottom.A Under-keel heightB Under-keel clearanceC Under-keel draftD Under-keel depth

All the following is true except that ______．

A.in many places a counter current flows in opposition to the main current close to the bank

B.current can vary with depth of water and large deep draught ships can experience different current effects at differing parts of the hull tend to turn to the wind

C.as speed is reduced，the increased proportion of the ship's vector which is attributable to current will set the ship close to obstructions

D.when close to the berth in a head current，there is a danger that flow inshore of the ship becomes restricted and the ship is subject to interactive forces

船舶在浅水域航行时，需要一定的富余水深，富余水深是指：

A.海图水深-船舶吃水

B.实际水深-海图水深

C.海图水深+潮高-船舶静止时最大吃水

D.海图水深+潮高-船舶静止时平均吃水

A ship under construction or major repair will acquire permanent magnetism due to ______ while sitting stationary in the earth's magnetic field．

A.hammering and jarring

B.vibration and pounding

C.varying magnetic fields

D.stable or permanent magnetism of the ship

The depth of the ship below the waterline measured vertically to the lowest part of the hull is called

A.trim

B.lean

C.draft

D.tonnage