单项选择题
In areas covered by Inmarsat HF can be used as an（）to satellite communications.
A.alternation B.alternate C.alter D.alternative
单项选择题 With the help of GMDSS（）can be alerted to a distress incident as soon as possible.
单项选择题 The Inmarsat system is open for use by（）countries on a non-discriminatory basis.
单项选择题 Within the polar areas it is（）to see a satellite in geo-stationary orbit .
