单项选择题
The main reason a long towline is used during an oceantow is that（）.
A.a margin of safety is provided should the line part B.the towline will wear more evenly C.there will be less stress on the towline D.a slight increase in speed will be realized
单项选择题 The forward movement of a ship in one revolution of its propeller is measured by（）.
单项选择题 The distance that a ship moves forward with each revolutionof its propeller，if there is no slip，is called（）.
单项选择题 The distance between rivets in a row is known as the（）.
