单项选择题
In using Loran-C，skywave reception gives greater range but is（）.
A.only accurate during daylight hours B.much less accurate C.only accurate at twilight D.more accurate than using ground waves
单项选择题 If the radio signal ground wave extends out for less distance than the minimum skywave distance，there is an area in which no signal is received．This is called the（）.
单项选择题 Automatic identification systems（AIS）are required to（）.
单项选择题 （）is not a component of GPS.
