什么是秘书学？秘书学主要研究哪些内容？

公文正文中用数字表示多层次结构时，正确的标识方法不包括（）。

公文的紧急程度、发文机关标识、发文字号等要素，位于公文的（）

Every deliberate act of wrongdoing by the master or any of the crew against the ship or cargo，without the authorization or privity of the Shipowner or his agent，is（）.

（）, or drum is used for hauling in or letting out the wires or ropes which will fasten the ship to the shore.

A salvor may be（）the whole or part of the payment due under Salvage Convention to the extent that the salvage operations have become necessary or more difficult because of fault or neglect on his part or if the salvor has been guilty of fraud or other dishonest conduct.

The （） is used for hauling in or letting out the wires or ropes.

Vessels must be（）valid certificate of nationality or certificated of registry or license.

When loading or discharging dry mud or cement，crew members should use goggles and（）.

逻辑运算符（AND、NOT、OR）的运算顺序是AND→OR→NOT。

公文的生效标识主要包括（）。

逻辑运算符（AND、NOT、OR）的运算顺序是AND→OR→NOT。