单项选择题
A.a message about the safety of navigation will follow
B.a message of an urgent nature about the safety of a ship will follow
C.the sender is in distress and requests immediate assistance
D.you should secure your radiotelephone
单项选择题 You have a large，broken-down vessel in tow with a wire rope and anchor cable towline. Both vessels have made provision for slipping the tow in an emergency; however，unless there are special circumstances（）.
单项选择题 You are underway and pass by a lighthouse. Its light，which was white since you first sighted it，changes to red. This means（）.
单项选择题 You are the first vessel to arrive at the scene of a distress. Due to the volume of traffic on the radio，you are unable to communicate with the vessel in distress. Which action should you take?（）