单项选择题 After sending a message describing the symptoms of an ill crew member，you receive a message containing the code MRL. This means “（）”.

单项选择题 After reporting a casualty to a mobile offshore drilling unit，which record must be retained on board? （）

单项选择题 After putting on a self-contained breathing apparatus，you open the air supply and hear a continuous ringing of a bell. What does this mean? （）