单项选择题
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（）.
单项选择题 A wooden float placed between a ship and a dock to prevent damage to both is called a（）.
单项选择题 A term applied to the bottom shell plating in a double-bottom ship is（）.