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（）.

Effects which are dutiable or subject to prohibitions or restrictions should be declared in （）.

逻辑运算符（AND、NOT、OR）的运算顺序是AND→OR→NOT。

PLC可进行远程的I/O控制，多台PLC之间可进行联网控制。

PLC控制系统是由PLC与用户（）连接而成的。

简述PLC的特点及PLC控制系统的发展趋势。

PLC的主要技术性能包括编程语言、（）、存储器、PLC的编程元件等，这些是PLC选型时的主要选择依据。

F1系列PLC—OR指令表示（）。

逻辑运算符（AND、NOT、OR）的运算顺序是AND→OR→NOT。