单项选择题
When liquid is free to move transversely in a tank，the effect is called（）.
A.free communication B.free density C.free surface D.negative GM
单项选择题 The term of Landfall means（）.
单项选择题 Preferred channel marks are colored with（）．
单项选择题 Preferred channel marks are aids to navigation which all following marks other than（）．
