单项选择题
A.only accurate during daylight hours B.much less accurate C.only accurate at twilight D.more accurate than using ground waves
A.Skip zone B.Blackout zone C.Diffraction zone D.Shadow zone
A.provide safety-related information automatically to shore stations，other vessels and aircraft B.receive safety-related information automatically from similarly equipped vessels C.exchange safety-related information with shore-based facilities D.All of the above