单项选择题
The ARQ mode should be applied（）.
A.when two stations communicate with each other B.in any radio system and at any time C.when one ship sends a message to another station D.for communication among some stations
单项选择题 It is required that ships proceeding at sea should keep continuous watch on（）.
单项选择题 General radio communications in the global system are those between（）concerning the management and operation of the ships and may have impact on their safety.
单项选择题 why do some stations keep silence?（）
