单项选择题
A.An atomic time standard
B.Daylight Savings Time
C.Eastern Standard Time
D.Greenwich Mean Time
单项选择题 The receiver uses（）satellites to compute latitude，longitude，altitude，and velocity．
单项选择题 The operator of the ship’s radiotelephone，if the radiotelephone is carried voluntarily，must hold at least a（）.
单项选择题 The operation of which aids to navigation may be suspended during war or national emergency（）.