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（）.
A.Skip zone B.Blackout zone C.Diffraction zone D.Shadow zone
单项选择题 Automatic identification systems（AIS）are required to（）.
单项选择题 （）is not a component of GPS.
单项选择题 （）is not an alarm required by IMO standards to be available on the ECDIS.
