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（）.

单项选择题 If EGC receiver shares a same antenna with Inmarsat SES, it means that it can receive MSI（）.