Answer: option (D.)They send signals in the gigahertz range to relays in the line of sight.
Explanation:
Microwaves are a form of electromagnetic waves with wavelengths between from about a meter to a millimeter; and they possess frequencies between 300 mega hertz(MHz )and 300 giga hertz(GHz.)
Microwaves travel by line-of-sight; unlike other radio waves of lower frequency. They do not bend around hills, nor follow the earth's surface as ground waves, nor reflect from the ionosphere .Therefore land microwave communication links are restricted only tovisual horizon to about 40 miles (64 km).
