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。

天然蓝宝石具有变色效应的原因是：（）

天然玉石

天然玉石

天然玉石是指（）。

属于天然玉石的是（）

天然玉石命名

逻辑运算符（AND、NOT、OR）的运算顺序是AND→OR→NOT。