单项选择题
What channel is general communication used?（）
A.any appropriate channel except public communication B.any appropriate channel except for public communications C.any appropriate channel besides public communication D.any appropriate channel beside public communication
单项选择题 General communications are those communications（）.
单项选择题 What’s the advantage when using the satellites?（）
单项选择题 Some radio waves（）the surface of the earth because of its（）.
