In a long-distance RF link, what statement about Fade Margin is true?

A: A Fade Margin is unnecessary on a long-distance RF link if more than 80% of the first Fresnel zone is clear of obstructions. B: The Fade Margin is a measurement of signal loss through free space and is a function of frequency and distance. C: Fade Margin is an additional pad of signal strength designed into the RF system to compensate for unpredictable signal fading. D: The Fade Margin of a long-distance radio link should be equivalent to the receiver’s low noise filter gain.