单项选择题
Channel 13 is primarily used for ship to ship communication. Channel 13 is also authorized for（）.
A.coast to aircraft operational communications B.aircraft to ship operational communications C.lock & bridge communications D.aircraft to ship navigational communications
单项选择题 Changing rescuers while carrying out artificial respiration should be done（）.
单项选择题 Carbon dioxide as a fire fighting agent has which advantage over other agents?（）
单项选择题 Camphor oil is classified as a（）.
