单项选择题
A.communications in the whole system will stop
B.the global system will be damaged
C.the back-up one will take its place
D.the whole system will not work
单项选择题 The satellites over the major ocean regions cover the globe（）.
单项选择题 （）is a space system using one or more artificial earth satellites.
单项选择题 The establishment of INMARSAT as an independent organization marked a great step forward for maritime radio communications. What does “marked a great step” mean in the sentence?（）