单项选择题
A.besides the polar regions
B.including the North Pole and the South Pole
C.above 70N and below 70S
D.as far north and south 70
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?（）
单项选择题 Why should the SES operators be very careful when the ship enters the area where three ocean regions overlap? Because（）.