Question Pool -- Category E2D

If an oscilloscope is connected to a TNC or terminal unit and is displaying two crossed ellipses, one of which suddenly disappears, what would this indicate about the observed signal?
A.
The phenomenon known as "selective fading" has occurred
B.
One of the signal filters has saturated
C.
The receiver should be retuned, as it has probably moved at least 5 kHz from the desired receive frequency
D.
The mark and space signal have been inverted and the receiving equipment has not yet responded to the change

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