单项选择题
PAN-PAN repeated three times over the radiotelephone indicates which type of message will follow?（）
A.Distress B.Safety C.All clear D.Urgency
单项选择题 In radiotelephone communications，the prefix PAN-PAN indicates that（）.
单项选择题 Fog forms when the air（）.
单项选择题 Five or more short blasts on a vessel’s whistle indicates that she is（）.
